BE: Allow project maintainers to enable item consumers directly

About

This issue is part of Reduce top-level group permission requirements ... (gitlab-org#20743), see that epic for more context. Also see the feature refinement planning issue #589457.

The backend will allow project maintainers to directly enable an AI Catalog item.

Proposal

This feature will use the new feature flag ai_catalog_project_level_enablement being added in !223681 (merged) for all changes. The flag will be scoped by current user.

At least in this iteration, we will continue to create the top-level group ItemConsumers, as projects within the same top-level group currently share a service account. This allows a smaller overall change for the feature. We can look at refactoring out the need for group ItemConsumers in a future refactor.

If a project maintainer enables an item, and there is no top-level group ItemConsumer created, we will create a group-level ItemConsumer for the first time, also.