Add TLS support to exec authenticator plugin
Add TLS support to exec authenticator plugin

https://github.com/kubernetes/community/blob/master/contributors/design-proposals/auth/kubectl-exec-plugins.md#tls-client-certificate-support

Allows exec plugin to return raw TLS key/cert data. This data populates
transport.Config.TLS fields.
transport.Config.TLS propagates custom credentials using
tls.Config.GetClientCertificate callback.
On key/cert rotation, all connections using old credentials are
closed

Kubernetes-commit: cd89f9473faa60c15b8e9d223e5c4f9dab53627a

kubernetes/apiserver,kubernetes/apiserver