Simplify/clarify extended key usage validation API

[djc]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.58%. Comparing base (d3b3870) to head (6ea5d18).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/verify_cert.rs	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #392      +/-   ##
==========================================
+ Coverage   97.51%   97.58%   +0.07%     
==========================================
  Files          20       20              
  Lines        4099     4100       +1     
==========================================
+ Hits         3997     4001       +4     
+ Misses        102       99       -3     

☔ 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.