Adds management project for a cluster

What does this MR do?

Adds new table and model to associate a cluster to a project for the purposes of:

• Selecting the managed cluster for a project as the highest priority in our existing cluster hierarchy
• The same as above, but EE multiple cluster scenario
• Providing cluster-admin privileges to the projects' CI jobs for a project's management cluster
• Query performance (don't make DeploymentPlatform do 3 queries when it can do 2 or 1)
• Add feature flag, default enabled
• Document feature!
• [-] Update via API, will do in follow-up MR

#32810 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers