SignalR SourceGenerator attribute API

## Background and Motivation

For PR https://github.com/dotnet/aspnetcore/pull/38025

We are adding a source generator for SignalR .NET clients to allow sharing strongly-typed interfaces between client and server. We have had the server side since first release and are now adding support for the client-side.

## Proposed API

```diff
namespace Microsoft.AspNetCore.SignalR.Client
{
+ [AttributeUsage(AttributeTargets.Method)]
+ public sealed class ClientHubAttribute : Attribute
+ {
+ }

+ [AttributeUsage(AttributeTargets.Method)]
+ public sealed class ServerHubProxyAttribute : Attribute
+ {
+ }
}
```

Package Name:
Microsoft.AspNetCore.SignalR.Client.SourceGenerator
Attribute DLL Name:
Microsoft.AspNetCore.SignalR.Client.SourceGenerator.HubProxyAttributes.dll

## Usage Examples

```csharp
internal static partial class ProxyExtensions
{
[ServerHubProxy]
public static partial T ServerHub(this HubConnection conn);

[ClientHub]
public static partial IDisposable CallbackRegistration(this HubConnection conn, T impl);
}

public interface IMyHub
{
Task GetNothing();
}

public interface IMyHubClient
{
Task ReceiveMessage(string message);
}

public class MyHubClient : IMyHubClient
{
public Task ReceiveMessage(string message)
{
Console.WriteLine(message);
return Task.CompletedTask;
}
}

var myHub = conn.ServerHub();
var dispose = connection.CallbackRegistration(new MyHubClient());
await myHub.GetNothing();
```

## Alternative Designs

HubServerProxyAttribute

HubClientProxyAttribute
ClientMethodsAttribute
SignalRClientAttribute
ClientRegistrationAttribute