Add generate_ssh_key, get_ssh_key, and revoke_ssh_key task types to
enable direct SSH access to an Edgebox instance. The generate task
creates an ED25519 key pair, installs the public key in authorized_keys,
and stores both keys plus the fingerprint in the option table. The
revoke task removes the key pair from disk, purges it from
authorized_keys, and clears the stored options.
