Layered Cookies integration

[annevk]

What is the issue with the Fetch Standard?

https://datatracker.ietf.org/doc/draft-ietf-httpbis-layered-cookies is the latest Cookies work at the IETF with the express goal of tackling httpwg/http-extensions#2084.

#1807 is the first step towards integrating that with Fetch. This will solve #693. It should also solve part of Storage Access' needs (though the remainder of that API will still need upstreaming to HTML).

Then we have these remaining pieces that all need some detailed attention:

• Cookie Store API integration. In particular due to the Cookie Store API having events, this comes with some difficulty.
• Clear-Site-Data integration. This should be mostly straightforward work, but might require new hooks in the Cookies section (which the first step PR will introduce).
• Service Workers integration. I believe some work is required here still, though not entirely sure.

Ideally we ensure that there's sufficient test coverage at each incremental point.