Retrieve upload instructions after the user has entered document metadata on the CDE

This endpoint returns the document upload information. The client retrieves this URL via the callback URL from the server after the user's completion of the upload metadata entry from a query parameter named upload_documents_url. In case when the user cancels the upload on the CDE UI, this callback will still be called, but the upload_documents_url query parameter will not be present, instead the server will provide a query parameter called user_cancelled_upload=true.