Implement list_messages function in Notification service.

src/service/notification.rs

@@ -95,4 +95,8 @@ 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());
+    }
 }
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