Add the selector message to the protobuf

[jhrozek]

Summary

Extends the Profile message with a nested message describing the
selectors.

Fixes: #3722

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

as part of a larger branch

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: 52.016% (+0.01%) from 52.002%
when pulling f31ba9d on jhrozek:selector_protobuf
into ff2be84 on stacklok:main.

[coveralls]

coverage: 51.837% (-0.005%) from 51.842%
when pulling e65c707 on jhrozek:selector_protobuf
into d521faa on stacklok:main.