Abstract out SSH configuration

[alanraison]

The code makes heavy reference to SSH keys and configuration. For several backends, it may make sense to connect in other ways (c.f. AWS Session Manager, Google Cloud Identity-Aware Proxy, Docker exec) which don't rely on an SSH server being directly accessible, or indeed being installed.

Perhaps a "Connection" plugin system could be extracted and imported where it makes sense, much like Ansible's model; https://docs.ansible.com/ansible/latest/plugins/connection.html and https://docs.ansible.com/ansible/latest/collections/community/aws/aws_ssm_connection.html