[Feature]: Oauth2 admin role mapping

Feature Summary

Add an option that automatically assigns users as administrators based on OAuth groups

What problem does this feature solve?

This allows to fully handle user management externally in a single location (IDP) and removes the necessity to create a local admin in the first place.

Proposed Solution

Every time when a user signs in, automatically add or remove the admin role from that user based on the value of a user configurable claim, e.g.:

OAUTH2_GROUP_CLAIM=groups
OAUTH2_ADMIN_GROUP=miniflux_admin

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have searched existing issues to ensure this feature hasn't been requested before.
• I understand that feature requests are not guaranteed to be implemented.
• I agree to follow the project's contribution guidelines.