Allow multi-hop SSH (like ProxyCommand)

I've received feedback from a handful of people that have complex setups, where they need to tunnel through multiple hosts to reach the database server. A typical setup might look like this:

[ Postico ] -- SSH -- [ Jump Host ] -- SSH -- [ Target Host ]

(The database runs on target host, but a firewall blocks direct SSH access, so we would need to tunnel the SSH connection through a jump host first)

This can be accomplished with OpenSSH using the ProxyCommand option. In a previous issue (#432) I've talked about how supporting ProxyCommand in general is not feasible, but the specific case of using multiple SSH tunnels might be feasible.

There are probably a number of ways to accomplish multi-hop SSH. I have a few questions for those of you who use such a setup:

1) Do you have access to all the keys/passwords required for the involved SSH servers, or do you need keys from the jump host to connect to the target host?

2) What does your setup look like? How many SSH connections do you need, and how many hosts are involved in total?

3) What's the exact ProxyCommand that you are using (for multi-hop SSH)?

Start with the multi-hop SSH setup described in the issue and review the answers to its three questions, especially the required credentials, host count, and ProxyCommand examples. The issue names no files, tests, or entry points; scope is complete only after a concrete implementation approach and expected connection behavior are defined.

Written by the indexing model from the issue text.