Add provider classes to API

[JAORMX]

Summary

Provided classes specify groups of configurations that work together for particular providers.

While the term was added to the DB and one part of the API, it wasn't hooked anywhere else.

This adds it where needed.

It also makes the db entry nullable, as the class is an optional parameter. That is,
we may have a provider that does not belong to any class... e.g. a self-configured provider.

Change Type

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• 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

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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.

[jhrozek]

The nullable class seems a bit confusing, but it was an explicit change, so I assume you had a good reason, so I'll approve so you can go ahead.