Provide an API for user and group mapping

Following CONFLUENCE-515 now we have a better API for the user and group formatting but we still have some specificity for confluence that we would like to refactor like the default group mapping and the rule which is applied on the user and group formatting if no formatter is defined.

This issue is to provide an API for this which can be used by the main migrator but also by the additional tools for the migration of data which is not migrated by the main migrator.