Automatic merge from submit-queue
allow group impersonation
Adds an "Impersonate-Group" header that can be used to specify exactly which groups to use on an impersonation request.
This also restructures the code to make it easier to add the scopes header next. This closely parallels the "Impersonate-User" header, so I figured I'd start easy.
@kubernetes/sig-auth
@ericchiang are you comfortable reviewing?
d07348dd84