Allow filter to be applied by database role

This is a feature request on behalf of a customer. The customer uses MaxScale filter to mask data, but woudl like to be able to specify not just applicable (or exempt) users, but also database roles.

To my untrained eye this seems possible, since roles are stored in MariaDB server and are applied using a SQL statement which goes through MaxScale - so MaxScale could be (or already is?) aware of the current role that the connected user has assumed (even if it is its default role) and adjust its behaviour accordingly.