Intent-to-ship: Early Hints Preconnect for Fx120

[Manuel Bucher]

We intend to enable Early Hints Preconnect in Fx120. Early Hints is the informational status code 103, allowing servers to send resource Link headers before the final HTTP response. An experiment on preconnect headers in Early Hints responses ran in release version 117 and 118 with overall positive results.

Bug: https://bugzil.la/1858712

Standard:

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

Platform coverage: all

Preference: Early Hints Preconnect feature is behind the pref "network.early-hints.preconnect.enabled". When set to true, we initiate the TLS handshake to resources specified by the link header in the Early Hints response by the server.

Intent-to-prototype: https://groups.google.com/a/mozilla.org/g/dev-platform/c/GNp44ffmGGs

Other Browsers:

- Blink: Shipped in 103 
- Webkit: Shipped in 17

Demo: Test websites simulating the effect of Early Hints Preconnect with an artificially delay between 103 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

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