Support for ssh-agent

[5nafu]

To be able to use the same ssh key on multiple computers, I have my key stored on a smartcard. Unfortunately, this does not leave me with an ssh private key file lying around on my disk and therefor, I cannot add my key to hetzner-kube.

As a workaround, I would need to create a separate key only for the management of my cluster and would need to copy this to all workstations in use.

It would be great, if there was an option in hetzner-kube to use the ssh-agent (maybe giving stating the path to a public key file).