Implement Http/2 CompleteAsync

[Tratcher]

#10886 Adding IHttpResponseCompletionFeature.CompleteAsync for HTTP/2 only.

• Allows for the graceful completion of a response message (from the client's perspective) without waiting for the request delegate to unwind.
• It makes sure all remaining response headers, data, and trailers are sent before it returns.
• It may throw if the response is in an invalid state such as a Content-Length mismatch.
• RequestAborted will not fire after this completes. Abort can still be used though.

Retrofitting this for HTTP/1.1 shouldn't be hard but I wanted to unblock the gRPC scenarios first.

While testing this I fixed a bug in response trailers where they were not getting cleared if the app threw an exception and the server converted it to a 500 response. You'd end up with a 500 + trailers.