support file url in $network.upload

[maks]

Currently $network.upload uses the mostly undocumented data property to upload a data uri to S3.
Instead, as discussed with @gliechtenstein on slack, it would be better to support using a local file url as the source of the data to be uploaded, which avoids having to hold the whole file being uploaded (which potentially can be very large) in app memory.
The syntax that will be used for the property is:

"data": {
  "file_url": "..."
}