should have an openssh backend

sshkit currently depends on 'net-ssh' for connecting to remote hosts. net-ssh is reimplementing ssh in pure ruby, and failing as badly as as one would expect on the security front: It still [supports cryptography that's obsolete and broken](https://github.com/net-ssh/net-ssh/issues/705), it does not implement current algorithms (in particular no post-quantum ones), it does not [support private keys on hardware tokens](https://github.com/net-ssh/net-ssh/issues/954), it does not check hostkeys by default, and probably lots more...

Therefore, sshkit should be able to call out to the "ssh" binary and thus leave the crypto to the professionals.