Add a "webtransport" request mode

[jan-ivar]

What is the issue with the Fetch Standard?

To solve w3c/webtransport#625, the idea is to look into reworking initialize WebTransport over HTTP to call fetch from the top.

The way fetch lets establish a WebSocket connection do this seems to involve a dedicated "websocket" request mode, among other things.

Would a place to start be to define a new "webtransport" mode?

cc @vasilvv, @nidhijaju feel free to chime in on what other hooks may be needed.

[annevk]

Yeah, I think that makes sense. There's very few issues reported against WebSockets's Fetch integration today: https://github.com/whatwg/fetch/issues?q=is%3Aissue%20state%3Aopen%20%20label%3A%22topic%3A%20websockets%22%20