Add option to pass signed urls to workflow

[cket]

Rather than download the redwood urls we should generate signed URLs and allow the pipeline to download them. This is necessary for autoscaling dockers.

To do this, I plan on modifying DockstoreRunner.download_and_transform_json to look at an argument along the lines of --generate_signed_urls. If that argument is passed, we can map the redwood url to an s3 url and use boto's generate_url function to create our signed url.