[SRI Message Signatures] Parse the subset of signatures valid for SRI.

This patch adds support for basic HTTP Message Signature parsing, as
per RFC9421. It does not support all (or even most) of that RFC's
options, limiting the accepted inputs to those that match the
constraints at https://wicg.github.io/signature-based-sri/#profile.

I expect we'll loosen those constraints over time, but this is a good
place to start.


This is part of a chain of CLs implementing this feature (#2 from
https://wicg.github.io/signature-based-sri/#overview):

1.  [Parsing] https://crrev.com/c/6020612 [You are here]
2.  [Validation 1] https://crrev.com/c/6030571
3.  [Validation 2] https://crrev.com/c/6032589
4.  [Enforcement] https://crrev.com/c/6038714

Bug: 379534943
Change-Id: Id4fd08c279cf59259ec1911675ceb45fd5679fca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6020612
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389286}