sources/oauth: add mastodon

[vitunvuohi]

Details

This PR is a draft that would close #4193

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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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[rissson]

Thank you for your contribution! Let me know if you want an early review or if I should wait for the PR to be marked as ready to review it.