Add provider store and provider manager code to create new providers

[jhrozek]

Summary

For both the provider auto-enrollment feature and for the OCI providers, we need
to create providers in the database. This PR implements the required provider store
and provider manager changes and will be followed by a patch to expose the functionality
through an RPC call.

Related: #3376

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 and as part of a larger patchset

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: 49.828% (-0.1%) from 49.955%
when pulling d77e9ee on jhrozek:provider_manager_create
into 2d7e6b3 on stacklok:main.

[jhrozek]

btw the test coverage will go up again in the next PR (testing some of the code here is actually more thorough through the RPC handler)