MariaDB Audit Plugin audits SET GLOBAL

The 'server_audit_events' system variable currently supports the following event types as filters:

• CONNECT
• QUERY
• TABLE
• QUERY_DDL
• QUERY_DML
• QUERY_DCL

I would like to request the following additional event type as a filter:

• GLOBAL_VARIABLES

If this event type is selected for audit logging, then whenever the SET GLOBAL command is executed, the audit plugin should log the following information to the audit log:

• The name of the system variable that is being changed.
• The old value of the system variable.
• The new value of the system variable.

Since the server_audit_% system variables can affect the audit logging configuration, it may be a good idea to make them special cases that are always logged, regardless of whether server_audit_events=GLOBAL_VARIABLES is set. See MDEV-17456 for more information about a previous problem related to that.