Implement access control (unit primitive) for Automated Merge/Squash Commit Messages

Goal

Restrict access and visibility of the feature to users who have Duo Enterprise.

Implementation

• Account for unit primitives [for help, talk to Nikola Milojevic]
  • Follow the guidance in https://docs.gitlab.com/ee/development/cloud_connector/#register-a-new-stand-alone-service, added by !154942 (merged), to add a new unit primitive for this feature.
  • Add/edit entry in the right YAML files
• Account for instance-level “enable AI features” [for help, talk to Pini]
  • https://docs.gitlab.com/ee/development/ai_features/#authorization (ai_global_switch)
• Account for seat assignment [for help, talk to Cloud Connector team]
  • https://docs.gitlab.com/ee/development/cloud_connector/#permission-checks
• Make changes to show/hide the feature based on the user's access

Note: this feature is standalone, i.e. NOT delivered as part of Chat