WebTransport PR follow-ups

[Daniel-Genkin-MS-2]

To avoid making the WebTransport PR even more massive and because of time constraints, I was not able to implement everything in it. This issue outlines some smaller follow up tasks that were identified during the PR's development and review. The larger items will receive their own issues. I have sorted the lists based on what, I think, is the importance/usefulness to the customers.

• Poison the message body and response body so users don't go using them
• Discard cancelled streams before users can accept them
• Add to docs and blog post
• Making webtransportsession a multiplexeconnectioncontext - not sure if it makes sense or not as there is a rather small overlap in functionality
• Server-initiated bidirectional stream partial closing - currently there is no way to close a single side of the bidirectional stream from the server. The JS client API does have a system for this. So, it might make sense to add it to the server-side as well.

Big features that have their own issues

• Performance optimizations for streams now that the WebTransport PR introduced new stream types #42789
• Adding datagrams support to WebTransport #42784
• Adding more tests to WebTransport #42786
• Adjusting the default Kestrel development certificate to support WebTransport #42787
• Implementing WebTransport over HTTP/2 #42785

[ByronAP]

What about .net client implementation not just javascript or did I miss that somewhere?

[Tratcher]

• Add support for WebTransport runtime#43641 - Client