contrib/site_rpc_client_lib: provide authorization_headers that allow authentication

In my environment, I am authenticating users against an LDAP server. The CLI
does not currently allow users to authenticate, though, so provide an example
site_rpc_client_lib that does.

Changes from v1:
     * Moved authorship info from __author__ attribute to docstring.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>