API Proposal: Make the abstract IpcEndpoint class public instead of internal

As of right now, the abstract class for [IpcEndpoint](https://github.com/dotnet/diagnostics/blob/main/src/Microsoft.Diagnostics.NETCore.Client/DiagnosticsIpc/IpcTransport.cs#L17) is marked as internal.
There are only two implementations of it (same file), including one that only has the processId as an entrypoint.
I have a couple of scenarios where I would like to reuse the existing code (the `IpcEndpoint` class at the minimum, the `PidIpcEndpoint` at best) but can't because the classes are internal.

My ask: Make the classes public, or at least derive from a public interface. Same thing for the DiagnosticClient constructor, it would be ideal if the `internal DiagnosticsClient(IpcEndpoint endpoint)` was public.

Thanks folks.