Add Clusters::KubernetesNamespace model

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8012

What does this MR do?

Introduce Clusters::KubernetesNamespace model. This one will be used to persist into database Kubernetes information, such as namespace, service account name and service account token

What are the relevant issue numbers?

First 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

NOTE

This merge request is the first step of is https://gitlab.com/gitlab-org/gitlab-ce/issues/5171 and is a pre-requisite for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22433 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22011