Machine ID: Generate "includable" ssh_configs

[strideynet]

Closes #46330

Adds a ssh_config and known_host per cluster:

• example.teleport.sh.ssh_config
• example.teleport.sh.known_hosts

These do not use the host matcher directive as the current configs do, this means that you are able to specify a host using it's bare hostname e.g "my-machine.foo" and with it's cluster appended hostname e.g "my-machine.foo.example.teleport.sh". This also opens the door to better supporting ProxyTemplates in the near future.

As these do not include the host matcher, it allows the user to define their own ssh_config and include this ssh_config using the Include directive, allowing for more flexible configurations.

➜  teleport git:(strideynet/flexible-ssh-config-output-tbot) ✗ ssh -F ./tbot-out/leaf.tele.ottr.sh.ssh_config noah@grus hostname                  
grus.net.stellar.haus
➜  teleport git:(strideynet/flexible-ssh-config-output-tbot) ✗ ssh -F ./tbot-out/leaf.tele.ottr.sh.ssh_config noah@grus.leaf.tele.ottr.sh hostname
grus.net.stellar.haus

# Cluster-specific ssh_config generated by tbot for cluster 'leaf.tele.ottr.sh' via proxy 'leaf.tele.ottr.sh:443'
UserKnownHostsFile "/Users/noah/code/gravitational/teleport/tbot-out/leaf.tele.ottr.sh.known_hosts"
IdentityFile "/Users/noah/code/gravitational/teleport/tbot-out/key"
CertificateFile "/Users/noah/code/gravitational/teleport/tbot-out/key-cert.pub"
HostKeyAlgorithms rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com
Port 3022
ProxyCommand '/Users/noah/code/gravitational/teleport/build/tbot' ssh-proxy-command --destination-dir='/Users/noah/code/gravitational/teleport/tbot-out' --proxy-server='leaf.tele.ottr.sh:443' --cluster='leaf.tele.ottr.sh' --tls-routing --connection-upgrade --resume --user=%r --host=%h --port=%p

changelog: Machine ID now generates cluster-specific ssh_config and known_host files which will always direct SSH connections made using them via Teleport.

[timothyb89]

Working well on my local cluster!

[public-teleport-github-review-bot]

@strideynet See the table below for backport results.

Branch	Result
branch/v15	Failed
branch/v16	Failed