Tracks Chromium-side work for https://github.com/WICG/webpackage/pull/360

- Update our structured header parser to draft-ietf-httpbis-header-structure-09
  - Support "List of Lists" parsing
  - Characters allowed in identifier (now called Token) are expanded.
    - This is necessary to allow header field names as tokens
- Implement the "Request matching" algorithm [1]
  - Determine which content negotiation mechanisms we support
    - [2] defines Accept, Accept-Encoding, Accept-Language. Probably Accept-Encoding doesn't make much sense in SXG loading
  - Implement the "Cache Behavior" algorithm [3]. Matching algorithms are different by content negotiation mechanism (See [2]).

[1] https://wicg.github.io/webpackage/loading.html#request-matching
[2] https://httpwg.org/http-extensions/draft-ietf-httpbis-variants.html#backports
[3] https://httpwg.org/http-extensions/draft-ietf-httpbis-variants.html#cache