[DataSync] Clean up `DataSyncHooks.ts`

At the moment, the types are too verbose in there and the file is difficult to grok. DataSync is always going to be somewhat complicated, but I think the developer experience can probably be improved there so that it's better at documenting itself.

This probably can go hand in hand with writing DataSync documentation - having clear types is should make it easier to understand how to use DataSync.

Also, maybe each DataSync hook should get a dedicated file.