Function probe_and_sign 

pub fn probe_and_sign(
    host: &dyn ProbeHost,
    cfg: &ProbeConfig,
    key: &SigningKey,
    now: u64,
    seq: u64,
    ttl_secs: u64,
) -> Result<SignedNodeFacts, NodeFactsAdvertError>

Probe host, build facts for the node owning key, and sign them.