External replication management in MariaDB Monitor

Add settings external_replication_monitor and external_replication_primary_role to MariaDB Monitor. external_replication_monitor defines a monitor that monitors the external cluster. external_replication_primary_role defines which server from the external cluster should be selected as external primary.

When external_replication_monitor is set and the external monitor runs, the local monitor ensures that the primary server of the local cluster replicates from the correct external cluster server. If a valid server is not found, the monitor removes the external replication connection. See MariaDB Monitor documentation for more information on external replication management.

Original description:
a feature where the mariadb monitor manages the external replication based on results of another monitor

• This feature needs to see the other monitor to see who is the master in that other monitor
• configurations to choose the primary from that other monitor or a slave from the other monitor
• one use case of grouping servers in different monitors could be in because they are in different data centers

its possible on the external cluster that the master im replicating from has gone down and I need to update the nodes in my monitor group need to update what node its replicating from.