Skip alerts if provider does not support it

[JAORMX]

Summary

The idea is to make artifact rules more re-usable. This detects if the
provider implements GitHub and if it doesn't it merely returns a no-op
alert engine. This is handy to be able to test artifact_signature rule
types with DockerHub or any othe provider that's not GitHub.

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

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: 51.357% (+0.005%) from 51.352%
when pulling 98fab72 on skip-alerts
into 949b313 on main.