Make PROXY protocol version configurable

Right now gitlab-sshd accepts PROXY v1 and v2 protocols, but there may be a case to restrict to v1 or v2 only.

Note that Google Cloud only supports v1 in their TCP load balancer: https://cloud.google.com/sdk/gcloud/reference/compute/target-tcp-proxies/create

AWS uses v2: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#proxy-protocol