[vnet] feat: write OpenSSH-compatible config file for VNet SSH

[nklaassen]

This PR is the next step in the implementation of VNet SSH (RFD).

VNet SSH now generates an OpenSSH-compatible config file and writes it out to ${TELEPORT_HOME}/vnet_ssh_config. Because the config block must only match subdomains of a Teleport cluster VNet SSH is handling, we must keep the config file up to date in case the user logs into or out of any clusters.

With this PR in place, VNet SSH works! I have tested it with tsh vnet and Connect, connecting with the OpenSSH client directly and in VSCode with the Remote-SSH extension. All the user still has to do manually is tell their SSH client to use the generated file. You can do with with ssh -F ${TELEPORT_HOME}/vnet_ssh_config or just once by adding an include to ~/.ssh/config. A subsequent PR will give the user the option to automatically add the include directive to their default SSH config file.

The demo is pretty simple, this just works with pnpm start-term and VNet running, to SSH to my server node-iot in my cluster called one.private. I even have per-session MFA turned on.

$ ssh -F '/Users/nic/Library/Application Support/Electron/tsh/vnet_ssh_config' node-iot.one.private
Nics-MacBook-Pro:~ nic$ echo "hello from vnet ssh"
hello from vnet ssh
Nics-MacBook-Pro:~ nic$ exit
logout
Connection to node-iot.one.private closed.

Parent PR: #55228

[rosstimothy]

Suggestion: move this within VNetSSHConfigPath?