Incorporates Kubernetes Namespace into Cluster's flow

What does this MR do?

Incorporates Clusters::KubernetesNamespace into clusters flow:

• Create the namespace/service account in GCP Kubernetes, when creating a cluster
• Create the namespace/service account in GCP Kubernetes, when adding a cluster
• Create the namespace/service account in GCP Kubernetes, when modifying namespace
• Replace KUBE_TOKEN and KUBECONFIG (passed to CI) with new credentials

What are the relevant issue numbers?

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

Manual QA

ABAC Cluster

• Add a Cluster
• Check namespace/service account were created on kubernetes side
• Install applications
• Run AutoDevOps pipeline

RBAC Cluster

Creating cluster

• Create Cluster
• Check namespace/service account were created on kubernetes side
• Install applications
• Run AutoDevOps pipeline

Auto DevOps for ABAC

• Switch to master branch
• Create a cluster ABAC
• Switch to this branch, restart GDK
• Run Auto DevOps pipeline

Auto DevOps for RBAC

• Switch to master branch
• Create a cluster
• Switch to this branch, restart GDK
• Run Auto DevOps pipeline

.gitlab/CODEOWNERS

@@ -6,7 +6,8 @@
 /doc/ @axil @marcia
 
 # Frontend maintainers should see everything in `app/assets/`
-app/assets/ @annabeldunstone @ClemMakesApps @fatihacet @filipa @iamphill @mikegreiling @timzallmann
+app/assets/ @ClemMakesApps @fatihacet @filipa @iamphill @mikegreiling @timzallmann
+*.scss @annabeldunstone @ClemMakesApps @fatihacet @filipa @iamphill @mikegreiling @timzallmann
 
 # Someone from the database team should review changes in `db/`
 db/ @abrandl @NikolayS
@@ -15,3 +16,5 @@ db/ @abrandl @NikolayS
 /ee/lib/gitlab/code_owners/ @reprazent
 /ee/lib/ee/gitlab/auth/ldap/ @dblessing @mkozono
 /lib/gitlab/auth/ldap/ @dblessing @mkozono
+/lib/gitlab/ci/templates/ @nolith @zj
+/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml @DylanGriffith @mayra-cabrera @tkuah