Migrate gitlab-shell discover command to Golang

Migrate the functionality saying hello to users through SSH to Golang:

❯ ssh git@gitlab.com
Welcome to GitLab, @reprazent!
Connection to gitlab.com closed.

This would include the first API call through unix sockets to /internal/discover.

This will need to support reading the API token from a file, and get optional HTTP Basic Auth username & password from the config.