Implement support for Related Origins Requests from WebAuthn 3 working draft

[Stormheg]

Fixes #42

Verified to work on Chrome v133 on macOS and Safari v18.3 using Caddy to emulate two different domains. Firefox has not added support yet.

Relevant piece of the spec: https://www.w3.org/TR/webauthn-3/#sctn-related-origins

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.0%. Comparing base (3ad1e62) to head (7aed169).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          13       13           
  Lines         720      748   +28     
  Branches       59       63    +4     
=======================================
+ Hits          720      748   +28     

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