Add WebSocket :protocol extension per RFC 9220.

[vi]

Attempt to add WebSocket support to h3.

I don't know any server that supports RFC 9220, so it is untested.

[pythonwood]

Hi. you can add some unit test.
support rfc9220 server:
https://github.com/ngtcp2/nghttp3

[vi]

What should the unit test test? as_str / from_str implementations?

[ararog]

Other implementation of RFC9220:

https://github.com/aiortc/aioquic

[vi]

Hi. you can add some unit test. support rfc9220 server: https://github.com/ngtcp2/nghttp3

It is appropriate to add a heavy dev-dependency for a such a trivial change?

[ararog]

Based on what I see on other implementations of websocket, h3 should only provide the minimum at http level to be able to establish websockets connection, which is, in short, the ability to set CONNECT pseudo-headers, and maybe send the settings frames, besides that, looks like the remaining parts can still take advantage of existing frame parsing libraries.

[vi]

Refreshed the pull request after a conflict from #273, which has a very similar idea.