Add support for compression dictionary transport

[pmeenan]

Add processing steps for handling HTTP Compression Dictionary Transport content encoding and dictionary negotiation (RFC pending publication).

This adds a processing layer between the HTTP cache and network fetch that handles most of the dictionary-based content encoding (including matching dictionaries to outgoing requests).

Additionally, it adds processing above the HTTP cache for storing the dictionaries for future use and defines the "compression-dictionary" initiator and destination (the matching HTML spec update is in-process).

Fix #1739

• At least two implementers are interested (and none opposed):
  • Chrome
  • Firefox
  • Safari
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/tree/master/fetch/compression-dictionary
  • PR for removal of "tentative": Removed tentative from compression dictionary tests web-platform-tests/wpt#54532
• Implementation bugs are filed:
  • Chromium: https://crbug.com/1413922
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1882979
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=295249
  • Deno (not for CORS changes): Add support for compression dictionary transport denoland/deno#30533
• MDN issue is filed: https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Compression_dictionary_transport
• The top of this comment includes a clear commit message to use.

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[pmeenan]

The RFC is pending publication so this will have to wait until that happens (should be any day now). RFC number has been assigned and final edits are complete: https://www.rfc-editor.org/auth48/rfc9842