Add VerifyProviderTokenIdentity to the GitHubProviderService

[jhrozek]

Summary

These two commits will allow to pass provider configuration during OAuth enrollment

Related #3263

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

this will be properly tested in a follow-up patch

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: 51.882% (+0.02%) from 51.867%
when pulling 5e0c1bf on jhrozek:verify_in_service
into 99f8dbe on stacklok:main.