Introduce citus_user and citus_admin roles

It would be useful if the extension created a citus_user role such that we can grant EXECUTE on relevant functions to users, without having to grant access to public or require the operator to set up new grants for every Citus release.

In addition, it would be useful to have a citus_admin role that can call administrative functions such as master_add_node. That way superuser is not required for those operations.