Fetch and store the language property

[jhrozek]

Summary

Since we already can get the language property in the REST call to get
most of the properties, we might as well store it as a property in the
database and let it be usable in selectors.

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

I really need to add automated smoke tests for the selectors, but for now I have used:

selector: repository.properties['github/primary_language'] == 'Go'

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: 53.722% (-0.006%) from 53.728%
when pulling 09ff2ca on jhrozek:single_language_prop
into 7f9febe on stacklok:main.