enable kubectl shell completion when its available

[bergerx]

Once kubernetes/kubernetes#105867 is merged, we can start using shell completions as told here:

https://github.com/kubernetes/kubernetes/pull/105867/files#diff-75e0556ae8d72c53672f08704f4338792f13d108e34330e57075c09c9f37be02R147-R171

Similar issue from cert-manager: cert-manager/cert-manager#4657

[bergerx]

This is now covered in https://github.com/kubernetes/enhancements/blob/master/keps/sig-cli/2379-kubectl-plugins/README.md#shell-completion-support

[bergerx]

The completion on flags is working out of the box for newer kubectl versions.

We should also enable completion for positional arguments as in kubectl get.