Content ex/im: asynchronous download/upload of data

The REST-based content export [1] does an export zip + + download within a single request. It can timeout easily, and does so on the Cloud platform

A solution would be to
a) make the locally created files from the Groovy-export [2] available for download, through another REST call
or
b) expand the REST service to first trigger the export and then download it, so in 2 separate calls.

Similar for uploading/importing of course.

[1] https://bloomreach-forge.github.io/content-export-import/cmd-example-export.html
[2] https://bloomreach-forge.github.io/content-export-import/tutorials-export-docs.html