cql: Also add `username` col. for LIST PERMISSIONS
cql: Also add `username` col. for LIST PERMISSIONS

the value for the `role` column is equal to the value for the `username`
column.

This change makes LIST PERMISSIONS backwards compatible with clients
that expect the `username` column to exist. This functionality also
exists in Apache Cassandra.

scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla