Display console messages, if available, in the Golang version of gitlab-shell

The following discussion from !287 (merged) should be addressed:

• @nick.thomas started a discussion:

  Since this applies to every command, we also need to support it in the golang version of gitlab-shell.

  Since it's feature-flagged and we only support discover and lfs_authenticate so far, while the CE MR happens to only add messages in the pull/fetch case, I'm OK merging this as-is. I'll create a follow-up issue though, and it would be great if you could tackle that too ;)

This is a feature added to the /internal/allowed API response in ruby this release, but not yet supported in the go commands.

cc @ashmckenzie