Add API response schema for datamap endpoint

[ssangervasi]

Docs Update Description

The OpenAPI docs for the Export Datamap endpoint /api/v1/datamap/{organization_fides_key} includes an example response, but little explanation of the structure of that response. The array is includes a header as the first element and the data as the second, which should be documented. Also, documentint the return type with pydantic class would allow OpenAPI to generate a TypeScript interface for the response, aiding clients.

Additional context

Definition of datamap response type/example: https://github.com/ethyca/fides/blob/1c26a67e3f1d02e495cba832c36ac59f212a09a6/src/fidesctl/api/ctl/routes/datamap.py#L31