[FR] Support getting relation details from the REST API's database row endpoints

[rafi-gh]

1~3 main use cases of the proposed feature I would like to build custom project management tooling using AppFlowy as the source of truth. One particular tool (PERT/CPM chart visualizer) would rely on the ability to create an activity/task dependency DAG using the relation field where each row in the database would have a list of parent dependencies. Currently support for relation fields in the row details endpoint has been disabled here for unknown reasons. Unlocking this feature would make the API rich enough to build this kind of specialized tooling.

what types of users can benefit from using your proposed feature Project managers that use AppFlowy as a data source and want to build their own tooling on top of it.

Additional context N/A