Create a direct file upload for a Multi FileFeed import

Creates a direct upload for a Multi FileFeed import. Returns an upload_id, a presigned_url, and a content_type. The client must PUT the file directly to the presigned_url, setting the returned content_type as the Content-Type header on the PUT request (the file store will reject the upload otherwise).

The direct upload flow consists of three steps:

1. Create (this endpoint): creates a tracking record and returns the presigned URL, content type, and upload ID.
2. Upload: the client uploads the file directly to cloud storage using the presigned URL.
3. Submit (direct-uploads/{upload_id}/submit): verifies the upload succeeded and links the file to the import.