API for instance-level kubernetes clusters

Customer ticket: https://gitlab.zendesk.com/agent/tickets/129534

We should have a way to manipulate instance-level clusters using the API, similar to the ones we have for project clusters or group clusters.

A customer is looking for a way to remove a cluster outside of the UI.

Definition of Done

• API should support CRUD for instance-level clusters
• Docs are updated to reflect changes

References

• https://docs.gitlab.com/ee/api/project_clusters.html#project-clusters-api
• lib/api/project_clusters.rb
• https://docs.gitlab.com/ee/api/group_clusters.html#group-clusters-api
• lib/api/group_clusters.rb