Improve Dev Proxy API port conflict handling

Raised by @t2kx

If you run Dev Proxy and port 8897 is already in use, you get an unhelpful error message.

We should update this to provide a resolution which is to change the default API port by adding the apiPort property to the config file to a custom value. In addition, if the developer is using Dev Proxy Toolkit, they will also need to update the API port extension setting to reflect the custom port number.

```
devproxy
info 1 error responses loaded from C:\Users\\AppData\Local\Programs\Dev Proxy\devproxy-errors.json
info New Dev Proxy version v0.29.0 is available.
See https://aka.ms/devproxy/upgrade for more information.
info Dev Proxy API listening on http://127.0.0.1:8897...
info Dev Proxy listening on 127.0.0.1:8000...

Hotkeys: issue (w)eb request, (r)ecord, (s)top recording, (c)lear screen
Press CTRL+C to stop Dev Proxy

fail: Microsoft.Extensions.Hosting.Internal.Host[11]
Hosting failed to start
System.IO.IOException: Failed to bind to address http://127.0.0.1:8897: address already in use.
---> Microsoft.AspNetCore.Connections.AddressInUseException: Only one usage of each socket address (protocol/network address/port) is normally permitted.
---> System.Net.Sockets.SocketException (10048): Only one usage of each socket address (protocol/network address/port) is normally permitted.
at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, Boolean disconnectOnFailure, String callerName)
at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
at System.Net.Sockets.Socket.Bind(EndPoint localEP)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateDefaultBoundListenSocket(EndPoint endpoint)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
--- End of inner exception stack trace ---
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportFactory.BindAsync(EndPoint endpoint, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.TransportManager.BindAsync(EndPoint endPoint, ConnectionDelegate connectionDelegate, EndpointConfig endpointConfig, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.<>c__DisplayClass28_0`1.<g__OnBind|0>d.MoveNext()
--- End of stack trace from previous location ---
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindEndpointAsync(ListenOptions endpoint, AddressBindContext context, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindEndpointAsync(ListenOptions endpoint, AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.ListenOptions.BindAsync(AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.EndpointsStrategy.BindAsync(AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.BindAsync(CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.StartAsync[TContext](IHttpApplication`1 application, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.Internal.Host.b__14_1(IHostedService service, CancellationToken token)
at Microsoft.Extensions.Hosting.Internal.Host.ForeachService[T](IEnumerable`1 services, CancellationToken token, Boolean concurrent, Boolean abortOnFirstException, List`1 exceptions, Func`3 operation)
fail: Microsoft.Extensions.Hosting.Internal.Host[9]
BackgroundService failed
System.Threading.Tasks.TaskCanceledException: A task was canceled.
at DevProxy.ProxyEngine.ExecuteAsync(CancellationToken stoppingToken)
at Microsoft.Extensions.Hosting.Internal.Host.TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
fail An error occurred while running Dev Proxy Exception Details: System.IO.IOException: Failed to bind to address http://127.0.0.1:8897: address already in use.
---> Microsoft.AspNetCore.Connections.AddressInUseException: Only one usage of each socket address (protocol/network address/port) is normally permitted.
---> System.Net.Sockets.SocketException (10048): Only one usage of each socket address (protocol/network address/port) is normally permitted.
at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, Boolean disconnectOnFailure, String callerName)
at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
at System.Net.Sockets.Socket.Bind(EndPoint localEP)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateDefaultBoundListenSocket(EndPoint endpoint)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
--- End of inner exception stack trace ---
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportFactory.BindAsync(EndPoint endpoint, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.TransportManager.BindAsync(EndPoint endPoint, ConnectionDelegate connectionDelegate, EndpointConfig endpointConfig, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.<>c__DisplayClass28_0`1.<g__OnBind|0>d.MoveNext()
--- End of stack trace from previous location ---
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindEndpointAsync(ListenOptions endpoint, AddressBindContext context, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindEndpointAsync(ListenOptions endpoint, AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.ListenOptions.BindAsync(AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.EndpointsStrategy.BindAsync(AddressBindContext context, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.BindAsync(CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.StartAsync[TContext](IHttpApplication`1 application, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.Internal.Host.b__14_1(IHostedService service, CancellationToken token)
at Microsoft.Extensions.Hosting.Internal.Host.ForeachService[T](IEnumerable`1 services, CancellationToken token, Boolean concurrent, Boolean abortOnFirstException, List`1 exceptions, Func`3 operation)
at Microsoft.Extensions.Hosting.Internal.Host.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
at DevProxy.CommandHandlers.ProxyCommandHandler.InvokeAsync(InvocationContext context)
fail ============ Inner exception ============ Exception Details: Microsoft.AspNetCore.Connections.AddressInUseException: Only one usage of each socket address (protocol/network address/port) is normally permitted.
---> System.Net.Sockets.SocketException (10048): Only one usage of each socket address (protocol/network address/port) is normally permitted.
at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, Boolean disconnectOnFailure, String callerName)
at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
at System.Net.Sockets.Socket.Bind(EndPoint localEP)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateDefaultBoundListenSocket(EndPoint endpoint)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
--- End of inner exception stack trace ---
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.TransportManager.BindAsync(EndPoint endPoint, ConnectionDelegate connectionDelegate, EndpointConfig endpointConfig, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.<>c__DisplayClass28_0`1.<g__OnBind|0>d.MoveNext()
--- End of stack trace from previous location ---
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindEndpointAsync(ListenOptions endpoint, AddressBindContext context, CancellationToken cancellationToken)
fail ============ Inner exception ============ Exception Details: System.Net.Sockets.SocketException (10048): Only one usage of each socket address (protocol/network address/port) is normally permitted.
at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, Boolean disconnectOnFailure, String callerName)
at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
at System.Net.Sockets.Socket.Bind(EndPoint localEP)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateDefaultBoundListenSocket(EndPoint endpoint)
at Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener.Bind()
```