Support for passphrase protected SSH keys

[Apreche]

Right now we only support SSH keys that are not encrypted by a passphrase. This is not as bad as it seems, as all secrets are stored in GitHub Secrets or Ansible Vault, which are both encrypted at rest. It would still be very preferable to support passphrase protected SSH keys.

We will have to allow the user a secure way to provide their passphrase, verify that it works, and store it securely for later use.