Signing: support SSH

[passcod]

Tracking/discussion issue for SSH key support.

(Add a 👍🏻 to this issue if you'd like this to happen.)

See https://www.openssh.com/txt/release-8.1:

ssh-keygen(1): add an experimental lightweight signature and
verification ability. Signatures may be made using regular ssh keys
held on disk or stored in a ssh-agent and verified against an
authorized_keys-like list of allowed keys. Signatures embed a
namespace that prevents confusion and attacks between different
usage domains (e.g. files vs email).

This is the same mechanism git uses to sign commits with ssh keys.