feat: abstract streamhub message notifications

[vaptu]

example

let notifier: Option<Arc<dyn Notifier>> = if let Some(httpnotifier) = &self.cfg.httpnotify {
    if !httpnotifier.enabled {
    None
    } else {
    // Or a custom-defined Notifier
    Some(Arc::new(HttpNotifier::new(
        httpnotifier.on_publish.clone(),
        httpnotifier.on_unpublish.clone(),
        httpnotifier.on_play.clone(),
        httpnotifier.on_stop.clone(),
    )))
    }
} else {
    None
};

The Notify trait can be customized by the users of the library.

[harlanc]

Thank you.