Implement pre-subscription hook

[Gsantomaggio]

Closes #71

• Implement the SubscriptionListener, during the Consumer creation, like:

 IConsumer consumer = await _connection.ConsumerBuilder()
            .Queue("MyStream").MessageHandler(
                async (context, message) =>
                {
....
                 }
            ).Stream().Builder().SubscriptionListener(
                ctx => { 
                 /// here the offset 
                 ctx.StreamOptions.Offset(<<myoffset>>); 
            }
            ).BuildAndStartAsync();

The listener function is called each time the consumer is created, even during the restart.

Check this test StreamConsumerBuilderShouldStartFromTheListenerConfigurationWhenConnectionIsKilled to see the listener in action.

• Refactor the code introducing ConsumerConfiguration, a helper class to hold the consumer configuration.