Intent to prototype: Early Hints

[Manuel Bucher]

Summary: Early Hints is an informational status code allowing servers to
send headers likely present in the final response. Early Hints is
enabled in nightly and beta since version 111. We're now experimenting
with Early Hints preconnect in release version 117 since 2023-09-18 and
expect to roll out afterwards. Early Hints preload will be experimented
and rolled out separately afterwards.

Bug: https://bugzil.la/1407355

Standard:

 - https://html.spec.whatwg.org/multipage/semantics.html#early-hints
 - https://www.rfc-editor.org/rfc/rfc8297

Platform coverage: all

Preference:

 - network.early-hints.preconnect.enabled true - enabling Early Hints
preconnect
 - network.early-hints.enabled true - enabling Early Hints preload

Other Browsers:

 - Webkit: testing preconnect in 17.1 technical preview
 - Blink: Shipped preload and preconnect since 103

Demo:

These test websites artificially add delay between 103 response and 200
responses:

 - preconnect (IPv6 only):
   https://lab.neon.rocks/1000/eh/images-preconnect-100.html
 - preconnect (less visible impact):
   https://lab.neon.rocks/1000/eh/images-preconnect.html
 - preload: https://lab.neon.rocks/1000/eh/images-100.html

Web-platform-tests: https://wpt.fyi/results/loading/early-hints