Add gitlab-managed option to clusters form

What does this MR do?

Add "gitlab-managed" option to Clusters form

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer

Closes #56557 (closed)

Backend - todo

• Add a new column to Clusters::Cluster. It could be:
  • Boolean managed. In this case, we need to default to true - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26675
• Rename current Clusters::Cluster#managed? method to user_provided?, so we can use managed? - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26567
• Add the new field in the view, pass it to the controller and ensure is saved in database. - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26096
• Ensure this field is not editable on the Cluster form after the cluster has been created
• If managed avoid creation of Kubernetes resources for GCP and user clusters.
  • When ci_preparing_state is not enabled
  • When ci_preparing_state is enabled
• Manual QA

Manual QA - When ci_preparing_state is not enabled

Clusters managed by GitLab

GCP-provided cluster	Resources created	User-provided cluster	Resources created

Clusters not managed by GitLab

Cluster managed by GitLab	Resources created	User-provided cluster	No Resources created

Manual QA - When ci_preparing_state is enbled

GCP Cluster managed by GitLab

Cluster	Resources before ADO pipeline	Pipeline	Resources after ADO pipeline	URL

GCP Cluster not-managed by GitLab

Cluster	Resources before ADO pipeline	Pipeline (fails due to missing namespace)	Resources after ADO pipeline