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.

Coverage is as good as I can get it. There are a few lines of unreachable code.

[codecov]

Codecov Report

❌ Patch coverage is 98.67550% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.47%. Comparing base (a3a6ef6) to head (9eb6880).

Files with missing lines	Patch %	Lines
http/unstable_message_signatures.ts	98.67%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7039      +/-   ##
==========================================
+ Coverage   94.43%   94.47%   +0.04%     
==========================================
  Files         630      631       +1     
  Lines       50566    51170     +604     
  Branches     8969     9164     +195     
==========================================
+ Hits        47750    48345     +595     
- Misses       2247     2249       +2     
- Partials      569      576       +7     

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[bartlomieju]

Thanks for the thorough implementation! A few things to address:

badge.svg changes don't belong in this PR — The reformatting of the SVG is unrelated to message signatures. Please drop it from the branch (or submit separately).

[tomas-zijdemans]

T%hanks for the feedback!

• Revert unrelated badge.svg change. Ooops! 🫣
• Fix silent ECDSA curve fallback
• Fix same class of bug for HMAC, RSA-PSS, and RSASSA-PKCS1-v1_5
• Document ;bs limitation: add warnings in JSDoc
• Add RFC citation for duplicate @query-param rejection. The throw is mandated by RFC 9421 section 2.2.8 ("the named query parameter MUST NOT be included")
• Remove redundant ?? undefined in verifyMessage.
• Add tests. Coverage can't reach 100% becuase of unreachable code