Add support for haproxy proxy protocol

Currently all ssh connections are from from localhost, and the original client address is not available to sshd.

The HAProxy proxy protocol allows the client address to be preserved for non-http connections like ssh.

tunnel origin request configuration should have an option to send a proxy header at the start of the connection. This would allow client address to be restored via [mmproxy](https://github.com/cloudflare/mmproxy).

Note this is already supported by spectrum (https://developers.cloudflare.com/spectrum/proxy-protocol).

Start by tracing the tunnel origin request configuration and connection setup to find where an option could control an initial proxy header. Confirm the expected HAProxy proxy protocol behavior and how mmproxy restores the client address. Done means the configuration can enable the header for non-HTTP connections without changing the default behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.