feat(ssh): add signed ssh certificates lab

[fxkk]

@FalcoSuessgott I've also taken a shot at creating a lab, does it look good or are you still missing something? :-)

[FalcoSuessgott]

nice PR, really like the documentation.

Just a few things:

• add the lab in the README, as well as in docs/home.md
• add a test scenario in tests/e2e.tftest.hcl, I believe you could simply run something like ssh <IP> cat <any-json-file>.json. Unfortunately, the ssh command must return valid json. You then can write simple assertions for that json file. If there isn't really any json file to output, you could probably just add one in docker_container.ubuntu.

Best,

Tom