Automatic merge from submit-queue

Impersonate user extra

Second commit builds on https://github.com/kubernetes/kubernetes/pull/30803.

This adds a restriction to `user.Info.Extra`, keys must be lower case.  This is because HTTP headers are case insensitive, so we can't be sure that we'll get the right case through proxies or even Go (the go library capitalizes after dashes).  I don't think anyone is using them, if they are, they'll need to update to properly plumb through an impersonation flow.

@kubernetes/sig-auth 
@ericchiang since you have background here.