In order to support expansion of custom report datasources and their entities, implement hooks to allow for:

1. Hook listeners to add entities to existing datasources;
2. Hook listeners to add elements (columns, filters, conditions) to existing entities

We require this for Workplace, because we effectively maintain exactly the same but without the benefits that the Hooks API brings (cleaner separation of code) - however it would also be beneficial for any other third-party code that wishes to extend custom reporting

Specification

TBD (this is a reminder/placeholder)