Refactor gitlab shell's gitlabnet client out of internal/

As part of gitaly#2731 (closed), we need to call the gitlab api. since gitlab shell already has a client, it'd be nice to just import that.

As discussed with @nick.thomas, our first step will be to move it out of gitlabshell's /internal package and keep it in gitlabshell for now, and gitaly can improt it.