Add authenticated user principal in Solr admin UI

When user is logged in to Solr's admin UI using Kerberos, no authentication info is displayed.
It would be very useful to see the logged in user principal. This could be specially crucial when SSO is being used and user not always aware that Solr is even configured with authentication mechanism.

Info should include:
1. user principal
2. mapped role (in case authorization plugin is also configured)