Consider using PING/PONG on WebSocket protocol level instead of explicit messages

[davidyuk]

https://github.com/aeternity/protocol/blob/master/channels/OFF-CHAIN.md#pingpong

As I understand, WebSocket server can setup PING/PONG frames exchange to ensure that the user is still connected, instead of doing it explicitly on user side

https://datatracker.ietf.org/doc/html/rfc6455#section-5.5.2