Launchpad MP (501141) - r00ta/openfga-groups-endpoints

[r00tabot]

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Commit message: feat: introduce v2 and v3 user groups endpoints.

Details:

• Implement servicelayer to CRUD user groups.
• Implement cascade logic to also delete openfga tuples when a group is deleted.
• Implement v3 endpoints
• Implement v2 endpoints and use directly the service layer to CRUD user groups.
• Introduce an alembic migration to add the new maasserver_usergroup table and pre-create the 2 default groups "administrators" and "users"
• Modify the existing openfga migrations to use the groups defined above and grant them permissions