Add "webtransport" request mode (integration)

[jan-ivar]

Add a "webtransport" request mode modeled on "websocket" to integrate WebTransport in a manner similar (but not identical) to WebSockets. Not exposed.

Fixes #1808.

• At least two implementers are interested (and none opposed):
  • Chrome, Firefox, Safari
• Tests are written and can be reviewed and commented upon at:
  • PR aims to be behavior neutral with existing tests in https://wpt.fyi/results/webtransport
• Implementation bugs are filed: none revealed as of yet from this change
• MDN issue is filed: None planned (fetch is under the hood; mdn websocket does not mention it either)
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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Preview | Diff

[jan-ivar]

Link to obtain a WebTransport connection blocked on w3c/webtransport#697.

Suggested change

	obtaining a WebTransport connection, given
	<a lt="obtain a WebTransport connection">obtaining a WebTransport connection</a>, given