Create Subscriber database and Subscriber repository struct skeleton.

src/repository/mod.rs

 pub mod product;
+pub mod subscriber;
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src/repository/subscriber.rs

+use dashmap::DashMap;
+use lazy_static::lazy_static;
+ise crate::model::subscriber::Subscriber;
+
+// Singleton of Database
+lazy_static! {
+    static ref SUBSCRIBERS: DashMap<String, DashMap<String, Subscriber>> = DashMap::new();
+}
+
+pub strict SubscriberRepository;
+
+impl SubscriberRepository {
+}
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