Make HubConnection more testable

[BrennanConroy]

This came up in the Hub Proxy source generator work, in order to easily test the source generator, we need to be able to mock the HubConnection.

The proposal is to add virtual to some of the public methods, highlighted below are the ones required for the source generator testing, but we might want to extend it to more of the public methods as well for general testability.

public class HubConnection : IAsyncDisposable
{
    Task StartAsync(CancellationToken cancellationToken = default);
    Task StopAsync(CancellationToken cancellationToken = default);
    ValueTask DisposeAsync();
+   virtual IDisposable On(string methodName, Type[] parameterTypes, Func<object?[], object, Task> handler, object state);
    void Remove(string methodName);
+   virtual Task<ChannelReader<object?>> StreamAsChannelCoreAsync(string methodName, Type returnType, object?[] args, CancellationToken cancellationToken = default);
+   virtual Task<object?> InvokeCoreAsync(string methodName, Type returnType, object?[] args, CancellationToken cancellationToken = default);
+   virtual Task SendCoreAsync(string methodName, object?[] args, CancellationToken cancellationToken = default);
+   virtual IAsyncEnumerable<TResult> StreamAsyncCore<TResult>(string methodName, object?[] args, CancellationToken cancellationToken = default);
}

[pranavkm]

API review - Let's make all of these APIs virtual. The names are somewhat unfortunate, but since they're already a public, we can't change them.