Stage a file submission by URL

The URL you specify will be deposited into the storage repository from a publicly accessible URL on the internet when the in-progress dataset is submitted. The dataset must be an in-progress dataset. The URL will be validated and metadata about it obtained to populate metadata about the file when it is added. Send the url to be retrieved as part of a JSON document like {'url': 'http://example.org/testing/my/file.csv' }. Priviliged users may have additional options to specify URL metadata rather than having live-validation and metadata gathering performed from the Internet.