Implement subscribe function in Notification service.

src/service/notification.rs

@@ -9,4 +9,11 @@ use rocket::http::Status;
 
 pub struct NotificationService;
 
-impl NotificationService {}
+impl NotificationService {
+    pub fn subscribe(product_type: &str, subscriber: Subscriber) -> Result<Subscriber> {
+        let product_type_upper: String = product_type.to_uppercase();
+        let product_type_str: &str = product_type_upper.as_str();
+        let subscriber_result: Subscriber = SubscriberRepository::add(product_type_str, subscriber);
+        return Ok(subscriber_result);
+    }
+}