Implement unsubscribe function in Notification service.

src/service/notification.rs

@@ -15,4 +15,17 @@ impl NotificationService {
         let subscriber_result = SubscriberRepository::add(product_type_str, subscriber);
         return Ok(subscriber_result);
     }
+
+    pub fn unsubscribe(product_type: &str, url: &str) -> Result<Subscriber> {
+        let product_type_upper = product_type.to_uppercase();
+        let product_type_str = product_type_upper.as_str();
+        let result = SubscriberRepository::delete(product_type_str, url);
+        if result.is_none() {
+            return Err(compose_error_response(
+                Status::NotFound,
+                "Subscriber not found".to_string(),
+            ));
+        }
+        return Ok(result.unwrap());
+    }
 }
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