Add Prefetch, Resource Hints and more Fetch keys

[autonome]

Draft, some of these need better home

Ok, a tangled web of features here. Easier to discuss in one place since they're interrelated.

Prefetch vs Speculation Rules

Speculation rules is broader. Prefetching is one purpose, but there can and will be more (eg prerender).

Developer POV seems to more be about "prefetch" than "speculation rules" as a singular thing, so I kept it split out - and that aligns w/ spec as well where it's separate.

Speculation Rules is more modern flexible approach that perhaps should be centered, but unless we're trying to actively shape how developers think about this, we should probably keep Prefetch as a standalone feature.

Resources hints - together or separate

A bunch of rel attribute values on the <link> tag related to performance, and collectively called resource hints in more than a few places, eg https://web.dev/learn/performance/resource-hints.

• Spec doesn't treat them as a group
• Caniuse has separate features for four of them

Bunched for now, feedback wanted on whether to group or not.

Update: modulepreload is separate feature already and a Caniuse feature so splitting this into separate features.

Authorization header removed in cross-origin redirects

I wiggled a bit on whether to put the authorization_removed_cross_origin keys all in their features or in Fetch, where that "feature" is specified.

1. Calling it a "feature" is a bit of a stretch

2. If you're a developer, you want to know whether network access method X supports feature Y

So I left the keys in the implementing network access method.

[autonome]

Ready for review, with a few specific bits to note:

• Caniuse for dns-prefetch starts at Chrome 4 but BCD has it starting at 46?!
• api.Request.duplex... maybe should be a feature
• two prefetch features, but are quite distinct so made them separate
• no biggie but modulepreload was done earlier and now stands out with a different feature key format from the other link-rel-* features

[ddbeck]

I've re-reviewed and the only thing outstanding now is the link-rel-preload description (#2418 (comment)). This is very close!

[autonome]

• Pushed the CORS/CORP/CORB keys over to Cross-Origin-Resource-Policy and X-Content-Type-Options HTTP headers #2455 so we don't lose that context.
• Updated the preload description
• Bundled all of prefetch into the link-rel-prefetch, which aligns w/ how we've handled key dependency in the past

[ddbeck]

OK, I have one line comment with suggestions for the new preload description, but otherwise this is good. Please merge after you've considered my suggestion.