Support user and group impersonation

[ktarplee]

For example the following are possible in other tools

• kubectl --as=cluster-admin ...
• helm --kube-as-user cluster-admin ...
• k9s --as=cluster-admin ...

The benefits are outlined nicely here and the k8s docs are [here]9https://kubernetes.io/docs/reference/access-authn-authz/authentication/#user-impersonation).

This does not work because when helmfile calls helm diff and it does not pass these arguments

helmDefaults:
   args:
   - "--kube-as-user cluster-admin"