Add background migration for Kubernetes Namespaces

What does this MR do?

Add background migrations to populate Clusters::KubernetesNamespaces.

What are the relevant issue numbers?

3rd part of https://gitlab.com/gitlab-org/gitlab-ce/issues/51716

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Backend - To do

• Make DB migration more naive by using raw sql instead. It'll make the background migration simpler and easier to test.

Note

This MR is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/51716 and it needs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22404 to be merged first.

After https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22404 is merged, target of this MR is going to be changed to master