Implement add function in Subscriber repository

src/repository/notification.rs

+use std::sync::RwLock;
+
 use lazy_static::lazy_static;
-use rocket::tokio::sync::RwLock;
 
 use crate::model::notification::Notification;
 
 lazy_static! {
-    static ref NOIIFICATIONS: RwLock<Vec<Notification>> = RwLock::new(Vec::new());
+    static ref NOTIFICATIONS: RwLock<Vec<Notification>> = RwLock::new(Vec::new());
 }
 
 pub struct NotificationRepository;
 
 impl NotificationRepository {
+    pub async fn add_notification(notification: Notification) -> Notification {
+        NOTIFICATIONS.write().unwrap().push(notification.clone());
 
+        notification
+    }
 }
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