Add PullRequest as available entity for selectors

[jhrozek]

Summary

This is more or less for completeness' sake. Both Ozz when reviewing the original code and Giuseppe when implementing the FE side of things were surprised that the selector API didn't handle PRs.

Fixes: #3988

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

mostly unit tests tbh

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 54.332% (+0.02%) from 54.316%
when pulling c72b86e on jhrozek:selectors_pull_request
into 0d7e9b8 on stacklok:main.