Implement list_all function in Subscriber repository.

src/repository/subscriber.rs

@@ -23,4 +23,16 @@ impl SubscriberRepository {
         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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