Use API calls to copy files, rather than reading and writing within the platform

When copying files on cloud storage it is likely to be much more efficient to use the azure/aws apis than to have a process reading a file and writing to a target. It is likely to scale much better, cost less and be more resilient.

E.g.
https://docs.microsoft.com/en-us/rest/api/storageservices/copy-blob
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CopyObject.html

This should be able to use the new alias section of the data plane to provide a mechanism for getting at the information needed to call the apis