Extract trait for ExtendedKeyUsage validation

[djc]

Intended to address the use case indicated by

• Introduce new XRequiredIfYPresent EKU policy with tests #372

@sgrams would this help you?

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.45%. Comparing base (4a03b51) to head (db64a7a).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/verify_cert.rs	94.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   97.52%   97.45%   -0.07%     
==========================================
  Files          20       20              
  Lines        4081     4088       +7     
==========================================
+ Hits         3980     3984       +4     
- Misses        101      104       +3     

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[cpu]

This LGTM, though it would be good to confirm it can replace #372 from the perspective of that user before we cut a release with the new API baked in.

[sgrams]

Hi, thank you, I think this helps!