Add audit plugin API functions to disable/enable specific audit plugins

When server_audit_logging=OFF is set, MariaDB Server's Audit Plugin API hooks still call the server_audit plugin's callback functions. This caused performance issues in MDEV-18661 / MDEV-19174.

To prevent issues like this, maybe it would make sense to add a couple audit plugin API functions that can be used to disable/enable specific audit plugins. For example:

bool mysql_audit_disable_plugin(plugin_ref);

bool mysql_audit_enable_plugin(plugin_ref);

That way, if the user sets server_audit_logging=OFF, then the server_audit plugin can tell the audit plugin API to disable its callback functions. And then if the user sets server_audit_logging=ON, then the plugin can tell the audit plugin API to enable its callback functions again.