Support stream-zip downloads

[patrick-austin]

Using something like https://github.com/uktrade/stream-zip, support zipping an entire download cart in one go. Not clear where the data being fed into this will come from. This is more similar to how the IDS currently behaves. A key requirement is that the relative structure of the data is maintained (i.e. Datafiles stay in their respective Datasets).

[patrick-austin]

With the latest implementation of S3, for public buckets the Datastore API is on the datapath by doing the S3 copy operation. As this is case, we could implement the zip here, if it becomes a firm requirement.