[Listener] Improve configurability

[Freyskeyd]

The Listener is responsible of listening for events that are appended to a stream and forward them to the SubscriberManager to notify every EventHandler interested.

The fact is that the Listener isn't really linked to the Storage. For now we are using PGNotify as a notifier but it could be any message queuing components such as kafka, etc.

In order to be able to configure the Listener of an application we need to move it into the EventStore crate where it belongs. We don't need to have a strong coupling between Listener and Backend because they can be different engines.

Listeners could be:

• A message broker
• Websockets exchange between applications
• HTTP calls between applications

The work on this is still in progress, feel free to share ideas or suggestions