Support keyPaths and keyDatas in sigstoreSigned policy

[jonesbusy]

Description

Fixes #789

Testing done

CI

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

❌ Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.30%. Comparing base (c5ffc83) to head (8dc4466).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/main/java/land/oras/policy/SigstoreVerifier.java	84.00%	2 Missing and 2 partials ⚠️
.../main/java/land/oras/policy/PolicyRequirement.java	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #798      +/-   ##
============================================
+ Coverage     86.86%   87.30%   +0.44%     
- Complexity     1238     1246       +8     
============================================
  Files            50       50              
  Lines          3996     4026      +30     
  Branches        543      550       +7     
============================================
+ Hits           3471     3515      +44     
+ Misses          311      296      -15     
- Partials        214      215       +1     

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