feat: Add Classlink as Federation Provider

[tonytoth]

Details

Add Classlink as Oauth provider for Federation login

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make website)

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