[k8s] Support exec based auth kubeconfigs on controllers

[romilbhardwaj]

GKE and EKS rely on exec based auth in the kubeconfig to authenticate with the k8s API server. This is currently not supported on SkyPilot controllers:

skypilot/sky/provision/kubernetes/utils.py

Lines 911 to 915 in ed4329a

	Using exec-based authentication is problematic when used in conjunction
	with kubernetes.remote_identity = LOCAL_CREDENTIAL in ~/.sky/config.yaml.
	This is because the exec-based authentication may not have the relevant
	dependencies installed on the remote cluster or may have hardcoded paths
	that are not available on the remote cluster.

The current suggested workaround is to create a kubeconfig that uses token based auth with a service account (generate_kubeconfig.sh).

This workaround introduces friction for users, and may not be be feasible in environments where users cannot create service accounts.

We should support exec based auth, maybe starting with supporting GKE and EKS. This would require installing relevant dependencies and copying over the cloud credentials.