[SRI Message Signatures] Support binary encoding for header components.

This CL implements binary wrapped HTTP field encodings, as specified in
https://www.rfc-editor.org/rfc/rfc9421.html#name-binary-wrapped-http-fields.
The implementation is incomplete, as the HTTP 1.1 line folding behavior
has already happened by the time we get to this point in the process,
but binary-wrapping the combined field value can still be helpful to deal
with charset issues, etc. Let's start with this and see if we need to
extend the behavior later.

Bug: 419441852
Change-Id: If07b127cecd388f97c4efa657243fc5ec275d606
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6861226
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1504924}