Make github webhook easier to extend with new events for auto registration.

[blkt]

Summary

The idea is to make it easier to extend github webhook with events that are tied to more than one repository, namely "installation" and "installation_repositories", which are necessary for repo auto registration.

Fixes #3359

Change Type

Mark the type of change your PR introduces:

• [ ] 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)
• [X] Documentation (updates or additions to documentation)
• [X] Refactoring or test improvements (no bug fixes or new functionality)

Testing

Work in progress.

Review Checklist:

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

[coveralls]

coverage: 52.365% (+0.3%) from 52.109% when pulling a01f93c3eb03d6ff25cd74eaf94120c0c01bffef on webhook-refactor into 5065d65f7ed76d5e00ae902df6ecc2733c198056 on main.

[jhrozek]

For the record, I like the code, I'm just doing some manual testing before finally acking.