add an async subscriber impl with thread pool

subscriber-notifs-async/include/app/services.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "app/inventory_manager.hpp"
+#include "core/subscriber.hpp"
+
+class SmsService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};
+
+class EmailService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};
+
+class LogService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};