Check fingerprint

[rukai]

Previously when manually connecting to an instance, ssh would pop up a message like this:

rukai@memes foo$ TERM=xterm ssh -i key [email protected]
The authenticity of host '18.204.219.189 (18.204.219.189)' can't be established.
ED25519 key fingerprint is SHA256:qGAC8ry5iMndcPb3lxy/o6PmoEA3gjCDk/zofJMP/lE.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])?

There was no way to actually check the fingerprint, so instead we would just accept it, leaving us open to MitM attacks.
This PR does the following:

• exposes public methods to retrieve the host public key in useful formats in case the user wants to connect with their own ssh client.
• alters the bash script emitted by ssh_instructions to perform checking of the fingerprint
  • unfortunately openssh is silly and the only way to do this is to create a temporary known_hosts file but it works