Implement add function in Subscriber repository.

src/repository/subscriber.rs

@@ -10,4 +10,14 @@ lazy_static! {
 pub struct SubscriberRepository;
 
 impl SubscriberRepository {
+    pub fn add(product_type: &str, subscriber: Subscriber) -> Subscriber{
+        let subscriber_value = subscriber.clone();
+        if SUBSCRIBERS.get(product_type).is_none() {
+            SUBSCRIBERS.insert(String::from(product_type), DashMap::new());
+        };
+
+        SUBSCRIBERS.get(product_type).unwrap()
+            .insert(subscriber_value.url.clone(), subscriber_value);
+        return subscriber;
+    }
 }
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