Implement list_messages function in Notification service.

src/service/notification.rs

@@ -94,5 +94,9 @@ impl NotificationService {
         let subscriber_result: Notification = NotificationRepository::add(payload);  
         return Ok(subscriber_result);  
     }  
-    
+
+    pub fn list_messages() -> Result<Vec<String>> {  
+        return Ok(NotificationRepository::list_all_as_string());  
+    }  
+
 }