Refactor downloading CI Artifact downloads using Gitlab-Workhorse-Send-Data

This would be a change in both gitlab-ce and gitlab-workhorse. The reason it is done differently is that Artifacts downloads were implemented before Gitlab-Workhorse-Send-Data was introduced.

It would also fix https://gitlab.com/gitlab-org/gitlab-ce/issues/19128 .

cc @zj @grzesiek