Add REPLICATION_SLAVE_ADMIN privilege so that MaxScale doesn't need SUPER privilege to STOP SLAVE or CHANGE MASTER

We currently require the granting of SUPER privilege to MaxScale user to allow stopping, restarting, and changing the slave. Some users object to the security risk of giving SUPER privilege for this, and would like it if MariaDB implemented something similar to MySQL's REPLICATION_SLAVE_ADMIN privilege. Since 8.0 MySQL has a series of these dynamic privileges that allow an account to manage a specific aspect of the server without opening up the account to everything. REPLICATION-SLAVE_ADMIN enables the account to connect to the master server, start and stop replication, and use the CHANGE MASTER TO and CHANGE REPLICATION FILTER statements.