Add proxy_download to perform proxied sending of all files

app/controllers/concerns/send_file_upload.rb

app/controllers/concerns/uploads_actions.rb

app/uploaders/object_storage.rb

config/gitlab.yml.example

config/initializers/1_settings.rb

doc/administration/job_artifacts.md

doc/administration/uploads.md

doc/workflow/lfs/lfs_administration.md

lib/api/helpers.rb

lib/api/job_artifacts.rb

lib/api/jobs.rb

lib/api/project_export.rb

lib/api/runner.rb

lib/api/v3/builds.rb

spec/controllers/concerns/send_file_upload_spec.rb