docs: add ADR for relay authentication

[nogringo]

Summary by CodeRabbit

• Documentation
  • Added a proposed architecture decision documenting relay authentication behavior.
  • Defined authentication modes for never authenticating, optionally authenticating, and requiring authentication.
  • Documented configurable asynchronous authentication handling, including refusal reasons and request details.
  • Added guidance for read and write authentication requests.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 38e50aa and 270cfdb.

📒 Files selected for processing (1)

• doc/library-development/ADRs/relay-auth.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/adr-relay-auth

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.66%. Comparing base (5a8cf9c) to head (270cfdb).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #739      +/-   ##
==========================================
- Coverage   72.44%   71.66%   -0.78%     
==========================================
  Files         227      227              
  Lines       13338    13338              
==========================================
- Hits         9663     9559     -104     
- Misses       3675     3779     +104     

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[frnandu]

sound good!