Replace the ruby runner with a go one?

The current runner is hard to set up, even with the Omnibus package because it is ruby software.

The runner is and will always be simple.

A runner written in Go will be much simpler to install. Can we adopt https://github.com/ayufan/gitlab-ci-multi-runner as an official GitLab project so people can use that instead?

/cc @dzaporozhets @marin @DouweM and @jacobvosmaer (sorry to discuss this during your vacation)