WebSockets over HTTP/2

[Tratcher]

Fixes #1375

Before this change the forwarder assumed that WebSockets always had to be forwarded using HTTP/1.1. Now it will honor the version and policy settings to decide which outgoing protocol to use. Request and response headers are adapted as needed. If HTTP/2 doesn't work then it can re-try with HTTP/1.1.

Notes:

• SocketsHttpHandler auto-flushes writes for HTTP/1.1 WebSockets, but not for HTTP/2. We have to manually flush writes.
• Something is hanging when YARP tries HTTP/2 prior knowledge to a non-TLS HTTP/1.1 endpoint. Hang when HTTP/2 WebSocket connects to a HTTP/1.1 non-TLS kestrel endpoint dotnet/runtime#80056
• HttpSys is used as a negative test server because it doesn't support H2WS. This requires TLS, so we use the ports registered for IIS Express's certificate.
• Added a lot of logging infrastructure
• Updated Docs

[MihaZupan]

Nice! I take it the telemetry for H2 WebSockets also "just work ™️" because ASP.NET exposes them the same way as H1?

[Tratcher]

Nice! I take it the telemetry for H2 WebSockets also "just work ™️" because ASP.NET exposes them the same way as H1?

Nope, had to add it there too 😁