Add a single binary to reduce binaries size

We currently have 3 binaries: gitlab-shell, gitlab-shell-authorized-keys-check and gitlab-shell-authorized-principals-check.

They all have duplicated code found in go/cmd/gitlab-shell, go/cmd/gitlab-shell-authorized-keys-check, and go/cmd/gitlab-shell-authorized-principals-check.

A suggestion from @nick.thomas in #206 (comment 203979135) is to have a single binary that accepts the executable name as an argument. Then we can have 3 small shell scripts that uses that and pass the arguments.