[whatwg/fetch] Return a `content-encoding` header for resource timing and more (PR #1796)

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The major change is to add `content-encoding` to response header list. This PR also adds description on how `content-encoding` is determined. (content negotiation) 

The purpose is to pass such value to resource timing. Further details are available at
https://github.com/w3c/resource-timing/issues/381.

Note: Per discussion at 12/05/2024 webPerWG call (https://docs.google.com/document/d/1mpFDrAWuV6IgvJ1KiL9sgIlcboC5uArtF8r_oqS1Sco/edit?tab=t.0#heading=h.af6v74wysf4m), we decided to allow arbitrary "content-encoding" value at "fetch". We only filter such value at client side, before passing the value to resource timing.

- [ ] At least two implementers are interested (and none opposed):
Likely,  Discussed and agreed at Feb 29, 2024 [W3C WebPerf call](https://docs.google.com/document/d/1qPPCtpg1MyVw3GGmd6VKZCdCyqoDub6Xgc8ANnq8SRI/edit#heading=h.wkdzwqaypyq6), Chromium already receiving content-encoding through fetch. But not sure about other browsers.

- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
[to be updated with a new link] https://chromium-review.googlesource.com/c/chromium/src/+/5958411

- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
Chromium: https://issues.chromium.org/issues/327941462
Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1886107
WebKit: https://bugs.webkit.org/show_bug.cgi?id=271632
Deno (not for CORS changes): …
[MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed:
https://github.com/mdn/content/issues/32823

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<a href="https://whatpr.org/fetch/1796.html" title="Last updated on Dec 13, 2024, 10:25 PM UTC (7647a50)">Preview</a> | <a href="https://whatpr.org/fetch/1796/0ce45ae...7647a50.html" title="Last updated on Dec 13, 2024, 10:25 PM UTC (7647a50)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1796

-- Commit Summary --

  * Add Content Encoding response header to fetch.
  * Add more about content-encoding
  * Remove a trailing space.

-- File Changes --

    M fetch.bs (77)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1796.patch
https://github.com/whatwg/fetch/pull/1796.diff

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