Add gitlab api access

split out from !2155 (merged) to make it easier to review.

This MR adds methods to access the gitlab api. It leverages gitlab-shell's client. It also modifies the Gitlab Test Server so it accepts Json as well as form data, which is what the gitlab internal api does.

This code essentially ports https://gitlab.com/gitlab-org/gitaly/-/blob/master/ruby/gitlab-shell/lib/gitlab_access.rb to go

it also modifies the test Gitlab Server to handle json