Refactor Clusters to be consisted from GcpProvider and KubernetesPlatform

What does this MR do?

Implements what was discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/39074

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/39074

The routes issue will be closed after we've done the following tasks.

I would like this MR to be as small as possible, only migrate gke_clusters, and then create follow up with migrating KubernetesService, and on top of that building orthogonal Adding apps.

cc @dosuken123 @grzesiek