WIP: Support Kubernetes RBAC for GitLab Managed Apps when creating new GKE clusters

What does this MR do?

See/merge MR for adding existing Kubernetes clusters first.

Enables support for users to create RBAC enabled clusters on GKE:-

• Allows users to install Helm into an RBAC cluster
• ditto with the other GitLab Managed apps.

This feature is hidden behind a feature flag.

To enable the feature flag, go to a rails console :

Feature.enable('rbac_clusters')

NOTE: Auto DevOps will not successfully complete in an RBAC-enabled cluster. This is planned to be supported at a later stage in this issue

What are the relevant issue numbers?

Closes #29398 (closed)

Development tasks

• Create new clusters with RBAC enabled (legacy_abac: false)
  • Create a new service account gitlab
  • Provide cluster-admin privs to the new service account (enough privileges to the new service account in order to perform operations such as create pods, querying namespaces, creating service accounts.)

Manual QA

• RBAC cluster
  • Check they can install cluster applications
  • Check that helper applications (tiller, ingress, etc) are still working as expected
  • Provide apps read access outside the namespace (if not provided by default)
• Check that ABAC clusters should still work
  • Check that we can install applications into an ABAC cluster still
  • Check that helper applications (tiller, ingress, etc) are still working as expected

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

Closes #29398 (closed)