Add support for opaque user impersonation

[sbose78]

When the admin wishes to update the user's profile, the admin console backend would be calling the Auth User API for the same.

However, to do so, the admin console backend would need to call the Auth User API with the user's token which it doesn't have.

Therefore, the backend would need to generate the same ( a very shortlived one ~ 1min ) and make the necessary API calls to accomplish the administration task.

[sbryzak]

@sbose78 could we possibly have the endpoint support both a user's access token and a service account token?

Edit: Actually of course this wouldn't work as we need to be able to identify the user in question.. how about another set of endpoints just for admin console?