Handle users deleted through the Keycloak management API, as well as through the Keycloak UI

[evankanderson]

Summary

It turns out that users deleted through the Keycloak API don't show up in the "User Events" queue, but instead show up in a different "admin events" queue. We should handle those deletions as well.

Fixes https://github.com/stacklok/minder-stories/issues/29

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

Testing

Manual testing and added a unit test. Note that the code generally follows the same form as user events, but with a different schema and different URL endpoint. I did not make a lot of effort to de-duplicate the two codepaths.

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.

[eleftherias]

I'll manually test this in a bit, note that there's a lint error in identity_events_test.go

[coveralls]

coverage: 52.96% (-0.02%) from 52.978%
when pulling bdb1af2 on evankanderson:cleanup-admin-delete
into 68d91f6 on stacklok:main.

[eleftherias]

Tested and works as expected when deleting a user from a different Keycloak client.

[evankanderson]

Fixed the lint and (in a different PR) fixed the data race that was causing tests to fail.