Make PROXY policy configurable
It would give us more flexibility when we decide to enable PROXY protocol
Related issue: #561 (closed)
Related discussion: https://gitlab.com/groups/gitlab-org/-/epics/8014#note_941734320
We've set the Policy to REQUIRED
here: !536 (merged)
If HAProxy changes are performed after gitlab-sshd has PROXY protocol enabled, we'll have a gap when gitlab-sshd rejects every request that doesn't have PROXY headers. We need to consider making this option configurable