feat: add sshkey pull

[amd-aliem]

Allow beacon to pull a public ssh key from dispatch after boot, and add to authorized_keys file in the test system.

• Corresponding dispatch change: feat: add ssh-key option dispatch#29
• This change works with versions of dispatch that does not provide any key and does not cause an error.
• For full functionality, the host image needs the openssh-server package.
• This does not do anything with guest images, probably can add a cloudinit script to configure that.