The *POST /sshkeys/sign* operation accepts a host name and a list of SSH
host public keys and returns a signed SSH host certificate for each key.
It uses the `step ssh certificate` command to sign the certificates,
which in turn contacts the configured *step-ca* service. This operation
will allow hosts to obtain their initial certificates. Once obtained,
the certificates can be renewed directly using the `step ssh renew`
command with the SSH private keys themselves for authentication.
