Create a "fake ssh" binary for proxies

Which part would be affected

The setup of rented servers once player LANs are added to GH
Explicitely, the servers intended as public proxies for the "SSH rental" mission

Description of the feature

This new binary would work as the "vanilla" ssh builtin command, with one difference
The binary would refuse to target a LAN IP

How would it enhance the experience?

As described here, a way to defend a private LAN without losing usability is to setup a single "admin-only gateway" with forwarded SSH, then to setup LAN-only (non forwarded) ssh for administrative access to the other machines
However, that means "public proxies" must either be placed on a separate LAN, or be sabotaged to make sure they can't reach the LAN from the inside

Checks to do

• is able to reroute a connexion to the WAN, in a completely undistinguishable way from the real ssh binary
• is not able to connect to a LAN ip
• is able to connect to another machine on the same LAN by providing its WAN address