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

pkg/util/webhook/authentication.go

@@ -196,6 +196,9 @@ func restConfigFromKubeconfig(configAuthInfo *clientcmdapi.AuthInfo) (*rest.Conf
 		config.Username = configAuthInfo.Username
 		config.Password = configAuthInfo.Password
 	}
+	if configAuthInfo.Exec != nil {
+		config.ExecProvider = configAuthInfo.Exec.DeepCopy()
+	}
 	if configAuthInfo.AuthProvider != nil {
 		return nil, fmt.Errorf("auth provider not supported")
 	}