Implement list_all_as_string function in Subscriber repository.

src/repository/notification.rs

@@ -16,4 +16,13 @@ impl NotificationRepository {
         NOTIFICATIONS.write().unwrap().push(notification.clone());
         return notification;
     }
+
+    pub fn list_all_as_string() -> Vec<String> {
+        return NOTIFICATIONS
+            .read()
+            .unwrap()
+            .iter()
+            .map(|f| format!("{}", f.clone()))
+            .collect();
+    }
 }