[whatwg/fetch] Add "webtransport" request mode (integration) (PR #1873)

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

Fixes #1808.

- [x] At least two implementers are interested (and none opposed):
   * Chrome, Firefox, Safari
- [x] [Tests](https://github.com/web-platform-tests/wpt) 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
- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed: none revealed as of yet from this change
- [x] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: None planned (fetch is under the hood; [mdn websocket](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API) does not mention it either)
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)


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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1873

-- Commit Summary --

  * Add "webtransport" request mode.

-- File Changes --

    M fetch.bs (38)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1873.patch
https://github.com/whatwg/fetch/pull/1873.diff

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