Support MongoDB as backend

[robertoschwald]

Currently we do not support Mongo as a logging backend. Therefore, one must use a 2nd SQL dataStore for audit logging. See #179

[flicken]

@robertoschwald What's the current state of supporting Mongo as a logging backend? Where places where changes need to be made? What's the rough effort involved?

I need auditing in a Grails 4 application with MongoDB, so instead of implementing everything by myself, I would consider modifying this plugin to support MongoDB as a back-end.