Support downloading packages directly to the remote machines rather than copying from host machine

In order to hook GET to omnibus-gitlab MR pipelines, one feature we need is to spin up an environment using the package produced by the omnibus-gitlab pipeline. Right now, the only way to do is to download the package to the host machine and then specify gitlab_deb_host_path which will make GET copy it to remote hosts and install it.

This means the package needs to be downloaded and reuploaded. This takes more time, and seems like a waste of computing resources. There might be more efficient ways to do it.

Proposal

Make GET accept a URL and a list of headers, using which it can download the packages directly to the remote machines and install it, instead of having it go through the host machine. So, with a GitLab API URL for downloading a package and a PRIVATE-TOKEN header, it can fetch the package directly from omnibus-gitlab MR pipeline.