Review design of Response class data field and flatten subtypes if possible

[robertbartel]

See this comment from #331 for a bit of context.

The Response class has a data field that implementations must provided. Prior to adopting Pydantic, data was just a dict. But the move to Pydantic required us to create full Pydantic BaseModel subtypes for data attributes. E.g., we end up doing things like creating a DatasetManagementResponseBody type just to fulfill data for DatasetManagementResponse.

On the surface, it would be nice if we could remove data from the abstraction and flatten the implementations. That would allow use to clean up certain parts of the code a great deal. However, the effects of such a change would be far reaching, and there could be something(s) that prohibits us from doing this. It is worth considering, but we must do so thoroughly and carefully.