Adjust the AutoRegistration protobuf message to be more FE friendly

[jhrozek]

Summary

Based on discussion this PR adjusts the AutoRegistration message to be
more FE friendly and clients-friendly in general. The main points are:
- it's easier for the clients to disable auto-registration for an
entity by flipping a boolean to false
- it's easier to extend with more attributes in the future

Related: #3266

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

unit tests

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: 52.393% (+0.01%) from 52.382%
when pulling fc67b57 on jhrozek:enroll_config_change
into f96e0ea on stacklok:main.

[blkt]

LGTM, but I suggest discussing UX with more people.