Implement add function in Subscriber repository.

src/repository/subscribers.rs

 use dashmap::DashMap;
 use lazy_static::lazy_static;
-use crate::model::subscriber::Subscriber;
+use crate::model::subscriber::{self, Subscriber};
+
+use super::product;
 
 lazy_static! {
     static ref SUBSCRIBERS: DashMap<String, DashMap<String, Subscriber>> = DashMap :: new();
@@ -9,4 +11,14 @@ 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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