Complete CRUD for group-level integrations

What does this MR do?

After adding the CRUD for group-level integrations !27557 (merged), we were missing the group_id column !38499 (merged), so an integration cannot belong to a group.

This completes the work, having now group-level integrations.

Group-level integrations are under a feature flag: group_level_integrations.

Related to #233932 (closed)