Machine ID: Leaf cluster support for SSH multiplexer

[strideynet]

Closes #43477

Modifies the SSH multiplexer protocol to support specifying a target cluster and modifies the generation of the known_hosts and ssh_config to include all accessible clusters.

The additional "|cluster" suffix added to the protocol is optional - if omitted the old behaviour of using the cluster that the bot has authenticated is used. This means that using a ssh_config generated by a previous version with a newer version of tbot will not result in an error.

However, the opposite is not true. Using a ssh_config generated by a newer tbot with an older tbot's tunnel will result in an error:

~/code/gravitational/teleport git:[master]
ssh -F ../teleport-scratch/tbot-ssh/ssh-proxy/ssh_config_old -p 13022 noah@macbook.root.tele.ottr.sh echo foo
failed connecting to host macbook:13022|root.tele.ottr.sh: failed to receive cluster details response
        failed to dial target host
        direct dialing to nodes not found in inventory is not supported
Connection closed by /Users/noah/code/gravitational/teleport-scratch/tbot-ssh/ssh-proxy/v1.sock port 0

We could roll to a v2.sock to make this error a little clearer since the socket wouldn't exist, but, they'd still get an error. Given that the ssh_config is generated by the tbot that hosts the tunnel, the only scenario where this could be encountered is if the user maintains a custom ssh_config and starts using cluster targetting with an older version of tbot. Additionally, the SSH multiplexer has only existed for 1 or 2 patch versions. This makes it feel to me like it's not worth rolling to v2.sock - but let me know your thoughts.

changelog: Added support for dialling leaf clusters to the tbot SSH multiplexer

[public-teleport-github-review-bot]

@strideynet See the table below for backport results.

Branch	Result
branch/v15	Failed
branch/v16	Failed