Expand profile handlers with selector support

[jhrozek]

Summary

Expands the profile handlers code with adding support for selectors so
that all the usual CRUD operations on profiles work with selectors, too.

The update code is not smart - I think that's OK for now. We can
optimize later when we see that selectors are modified often.

Fixes: #3723

Change Type

• 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 + 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: 54.25% (+0.06%) from 54.187%
when pulling 3e2fc97 on jhrozek:selectors_handlers
into b3b3256 on stacklok:main.