feat(http/unstable): add RFC 9421 message signatures

[tomas-zijdemans]

Add HTTP Message Signatures (RFC 9421) for signing and verifying HTTP requests and responses. Builds on the structured fields module (RFC 8941) already in std.

[codecov]

Codecov Report

❌ Patch coverage is 93.73942% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.87%. Comparing base (48bd522) to head (035e57c).

Files with missing lines	Patch %	Lines
http/unstable_message_signatures.ts	93.73%	21 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7039      +/-   ##
==========================================
- Coverage   93.88%   93.87%   -0.01%     
==========================================
  Files         627      628       +1     
  Lines       49995    50586     +591     
  Branches     8794     8974     +180     
==========================================
+ Hits        46936    47490     +554     
- Misses       2478     2499      +21     
- Partials      581      597      +16     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.