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 93.89%. Comparing base (48bd522) to head (c75d02d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7037      +/-   ##
==========================================
+ Coverage   93.88%   93.89%   +0.01%     
==========================================
  Files         627      628       +1     
  Lines       49995    50129     +134     
  Branches     8794     8824      +30     
==========================================
+ Hits        46936    47071     +135     
  Misses       2478     2478              
+ Partials      581      580       -1     

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