Initial websocket notifications experiment

Implement minimally needed WebSocket protocol to be able
to read incoming notification messages from a backend.
Add a test to set up a backend and enable notifications with:

settings: {
    websocket: true # this enables websocket notifications
}

For now the notification json are printed, in the future the
test should check the validity of the notification data.

This only works with ENGINIO_API_URL=https://staging.engin.io