Implement list_all function in Subscriber repository.

src/repository/subscriber.rs

@@ -19,4 +19,12 @@ impl SubscriberRepository {
         SUBSCRIBERS.get(product_type).unwrap().insert(subscriber_value.url.clone(), subscriber_value);
         return subscriber;
     }
+
+    pub fn list_all (product_type: &str) -> Vec<Subscriber> {
+        if SUBSCRIBERS.get(product_type).is_none {
+        SUBSCRIBERS.insert(String::from(product_type), DashMap::new());
+        };
+
+        return SUBSCRIBERS.get(product_type).unwrap().iter().map(|f| f.value().clone()).collect();
+    }
 }
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