Refactor KubernetesCluster to base and project

What does this MR do?

Previously, using Resource::KubernetesCluster.fabricate! would fabricate a Project Cluster. Currently there is no way to differentiate between Project, Group, and Instance-level clusters.

This MR creates a base class and adds a ProjectCluster class.

• resource/kubernetes_cluster/base.rb
• resource/kubernetes_cluster/project_cluster.rb
• (resource/kubernetes_cluster/{group|instance}_cluster.rb)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content