Retrieve the user's selection from the CDE

This endpoint returns the document selection. The client retrieves its url via the callback url from the server after the selection has finished. The url of this endpoint is retrieved from the callback sent to the client in a query parameter named selected_documents_url. In case when the user cancels the selection on the CDE UI, this callback will still be called, but the selected_documents_url query parameter will not be present, instead the server will provide a query parameter called user_cancelled_selection=true.