Add a CreateProvider rpc handler

[jhrozek]

Summary

Adds a Server.CreateProvider() handler that implements the already
existing grpc method. Adds tests.

Fixes: #3376

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

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

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: 51.878% (+0.5%) from 51.352%
when pulling cf61638 on jhrozek:create_provider_rpc
into 949b313 on stacklok:main.

[dmjb]

Nice, I love how little logic is in this handler :)