feat: external managed cluster registration and node handling

Hello fellas,

Since cloudstack kubernetes provider is successor of cloudstack cloud controller, shouldn't it also allow to register external managed clusters automatically? I am not saying it should be mandatory, but would be great if we had a choice. So it would fetch information about nodes and register cluster to the cloudstack, besides that it could handle node removal if virtual machine is removed and so on.

The issue names no files, tests, or entry points. First clarify the expected external-cluster registration and node-removal behavior, then locate the provider's existing cluster and node handling paths. Done should include an agreed scope, implementation, and tests covering registration and removal.

Written by the indexing model from the issue text.