Add role support for monitors and services

Add role support for monitors and services. If a monitor or service are configured with a role, the monitor or service will activate that role when connecting to the backend. For monitors, the role applies to normal monitor connections by MariaDB Monitor and Galera Monitor. replication_user is not affected. For services, role is activated when fetching user accounts, not for more specialized features like Binlog Router replication.

Original description: When assigning provileges to the user that is used for monitoring, to fetch authentication data and for replication it would be convenient to be able to use database roles. As it stands, this only works if the role in question is the default role for the user in question. I suggest that the settings for user/password for servers and sservices, monittoruser/monitorpw for servers and replication_user and replication_password for the mariadbmon monitor are complemented with a similar role parameter. When a role is specified, the role in question would be anabled after connection, i.e. by running SET ROLE <rolename>.