Add github/pull_author_login as a property for PRs

[jhrozek]

Summary

This allows us to write selectors like:

selection:
  - entity: pull_request
    selector: pull_request.properties['github/pull_author_login'] != 'jhrozek'
    comment: "We trust Jakub. Don't check his PRs"

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

manually

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: 52.821% (-0.002%) from 52.823%
when pulling 6d55224 on jhrozek:pr_prop_author_name
into 31a94e0 on stacklok:main.