feat(sync): add labels to kubeconfig file

[onuryilmaz]

With the PR cloudoperators/greenhouse#1357 kubeconfig resources will have labels based on the cluster data.

This PR, adds these labels to kubeconfig file under extensions:

- cluster:
    certificate-authority-data: LS0tLS1...
    extensions:
    - extension:
        cluster: demo-cluster
        cluster-type: demo
      name: labels
    server: https://api.demo.greenhouse...