allow repo to override tracked?/1 to avoid specifying all tracked schemas

[superchris]

In the code, there is a function tracked? which should allow a given Repo to provide alternate behavior. However, the tracked_schemas config setting is referenced several other places, most importantly in ExAudit.Tracking.compare_versions/3 which results it in not being possible to customize the behavior to track all schemas by default. This function probably could just skip the check, as it appears that Repo will only attempt to track changes if tracked?/1 returns true. Could probably PR this if this approach makes sense