Add adbreak_notify event

[Luzifer]

As a streamer I want to give my viewers a little heads-up in the chat when an ad-break is about to start.

To do so we can't rely on the EventSub events as those only trigger when the ads are already running.

Instead we should periodically poll the Get Ad Schedule API to know when the next ad will be playing. The user should be able to configure the time in advance used to send the notification using the module config:

module_config:
  adNotify:
    default:
      notifyAdvance: 1m
    luziferus:
      notifyAdvance: 2m

which would then create an adbreak_notify event in #luziferus 2m before the ads are starting (by schedule, manually created ads will obviously not have that event) and in all other channels 1m before.