Adds observer to receive callbacks for pong events.
Iterates through each observer, so that their notification functions may be called.
Removes observer to stop receiving callbacks for pong events.
Starts sending pings on an interval and receiving pongs.
Stops ping-pongs.
[[PingPong]] sends ping-pongs across the signaling connection via the [[SignalingClient]]. It measures the round-trip time between the client ping and server pong and acts as a heartbeat to keep the underlying WebSocket connection alive.