Implements `WebSocket` over http/2. This requires a conformant http/2
server supporting the extended connect protocol.
Passes approximately 100 new WPT tests (mostly `?wpt_flags=h2` versions
of existing websockets APIs).
This is implemented as a fallback when http/1.1 fails, so a server that
supports both h1 and h2 WebSockets will still end up on the http/1.1
upgrade path.
The patch also cleas up the websockets handshake to split it up into
http, https+http1 and https+http2, making it a little less intertwined.
This uncovered a likely bug in the WPT test server:
https://github.com/web-platform-tests/wpt/issues/42896
Use new https://github.com/denoland/rustls-tokio-stream project instead
of tokio-rustls for direct websocket connections. This library was
written from the ground up to be more reliable and should help with
various bugs that may occur due to underlying bugs in the old library.
Believed to fix #20355, #18977, #20948
