Introduce a `send-url:` method that allows to serve remote HTTP/GET file, like S3-based file
This makes it possible to fulfill requirements of having an Object Storage being hidden behind a Workhorse.
This MR does support content ranges and etags. It has a curated list of headers that are passed to the proxied server.
This is needed for: https://gitlab.com/gitlab-com/infrastructure/issues/3618, where in order to support old Runners we have to proxy the request instead of performing direct upload.