Add spec for No-Vary-Search hint in speculation rules

[kjmcnee]

We introduce the "expects_no_vary_search" field for speculation rules and specify how it is parsed.

When matching prefetch records, we now have it:

• perform inexact matches based on the response's No-Vary-Search value
• block on the availability of headers for potential matches
• have the identification of potential matches incorporate the No-Vary-Search hint

Note that this spec text describes blocking on any available prefetch. The current chromium implementation blocks only on a single ongoing prefetch.

[github-actions[bot]]

Preview: - [no-vary-search.html](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/head/no-vary-search.html) ([diff](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/diff/no-vary-search.html) [base](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/base/no-vary-search.html)) - [prefetch.html](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/head/prefetch.html) ([diff](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/diff/prefetch.html) [base](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/base/prefetch.html)) - [speculation-rules.html](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/head/speculation-rules.html) ([diff](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/diff/speculation-rules.html) [base](https://storage.googleapis.com/spec-previews/WICG/nav-speculation/pull/278/base/speculation-rules.html))