Feature/system assignment group

[konac-hamza]

#757 System group role assignment API

This PR covers CRUD operations for system role assignments of group actors.

• API endpoints and test methods are mirrored from the system user role assignment implementation
• OPA policy rules are mirrored from the system user role policy package
• Integration tests are implemented for all endpoints
• The following bug is fixed in a separate commit:

When listing roles for a system group, the API was returning all system role assignments in the database instead of only those belonging to the requested group. The root cause was that group_id was never added to the actors filter in list_assignments, while user_id was handled correctly. With an empty actors list, the database query had no filter and returned every system assignment. The fix was adding group_id to the actors list the same way user_id is handled.