Add PROXY v2 support for tracking client IP addresses

As discussed in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10954#note_436793419, any load balancer in front of SSH will obscure the client IP address.

If we build in a SSH server as in !394 (merged), we can potentially copy the approach in gitlab-pages!278 (merged) to get the real IP address from the load balancer.