Feat/OIDC free auth

[MagicRB]

Description

This PR implements a generalized OIDC auth backend which is largely a copy-paste from the Gitea on. This needs cleanup and feedback on if these changes are desired, opening for the sake of sharing.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Improvement (change that would cause existing functionality to not work as expected)
• Code refactoring
• Performance improvements
• Documentation update

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