Description of the problem, including code/CLI snippet

Certain end points like SideKiq are skipped by CLI parser because they don't manage any objects. This is because only RESTManager classes that defined the _obj_cls attribute are parsed for the endpoints.

I described this behaviour here: #3083 (comment)

Expected Behavior

All endpoints with managers are accessible.

Actual Behavior

Missing endpoints:

• group-registry-repository: /groups/{group_id}/registry/repositories
• project-audit: /projects/{project_id}/audit_events
• project-iteration: /projects/{project_id}/iterations
• sidekiq: None
• user-identity-provider: /users/{user_id}/identities

This list is made by taking a diff between current main branch python -m gitlab --help and my quick experiment of adding all RESTManagers to the CLI parser.