Automatic merge from submit-queue

docs/proposal: add proposal for kubectl login

This PR updates https://github.com/kubernetes/features/issues/32 and https://github.com/kubernetes/kubernetes/pull/25758 by adding a proposal for a "kubectl login" command.

It's a bit more involved than the implementation discussed with @deads2k in #25758, by proposing a long term goal for the overall subcommand.

cc @kubernetes/sig-auth @kubernetes/kubectl