Function peer_signal_content 

pub fn peer_signal_content(topic_id: &str, node_addr: Option<&str>) -> String

The kind-3310 peer-signal content, shared by every community transport (the v1 channel plane and the v2 chat plane must stay byte-compatible): {"op":"ad","topic":..,"addr":..} advertises an Iroh node, {"op":"left",..} departs.