Presigned links (valid for 3 hours, or 1 hour for files in external
buckets) for the files in a selection, with their folder paths, for
the Pennsieve agent to download in parallel. The route for selections
too large to download as one archive.

Without nodeIds the selection is the whole dataset. Files come in
pages of up to 1,000, in file id order: pass a page's next as the
following request's cursor until there is no next. Each page is
signed when requested. The header counts the whole selection.