webhook: support exec auth plugin
webhook: support exec auth plugin

This allows webhook static kubeconfig to use an exec auth plugin to
configure authentication.

https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#authenticate-apiservers

Kubernetes-commit: 8b45296661de1f12f441736ae44895ac30f320d4

kubernetes/apiserver,kubernetes/apiserver