feat(http/unstable): add RFC 9530 digest fields

[tomas-zijdemans]

NOTE: uses toBytes, so connects to this PR. I will update as necessary.

Adds Content-Digest / Repr-Digest creation and verification. These headers let you hash a request body and verify it wasn't tampered with in transit.

RFC 9530 (IETF standards track, Feb 2024) is the modern replacement for the old RFC 3230 Digest header. It's a building block for HTTP Message Signatures (RFC 9421), where Content-Digest is typically a signed component binding the signature to the body.

The module is small and browser-compatible, reusing existing @std primitives (timing-safe-equal, unstable-structured-fields, unstable-to-bytes).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.39%. Comparing base (d2fb2d6) to head (6c5e104).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7037      +/-   ##
==========================================
+ Coverage   94.38%   94.39%   +0.01%     
==========================================
  Files         628      629       +1     
  Lines       50178    50312     +134     
  Branches     8840     8870      +30     
==========================================
+ Hits        47360    47493     +133     
  Misses       2251     2251              
- Partials      567      568       +1     

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[tomas-zijdemans]

Updated to use the stable toBytes