add an async subscriber impl with thread pool

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

@@ -0,0 +1,25 @@
+
+#pragma once
+
+#include <memory>
+#include <string>
+#include <unordered_map>
+
+#include "core/subscribable.hpp"
+#include "core/thread_pool.hpp"
+
+using ProductCount = int;
+using ProductId = std::string;
+
+// inventory manager is not generic ( works with the specific types ProductId
+// and ProductCount ) but the subscribable is generic
+class InventoryManager : public Subscribable<ProductId, ProductCount> {
+   private:
+    std::unordered_map<ProductId, int> inventory_;
+    std::shared_ptr<ThreadPool> thread_pool_;
+
+   public:
+    explicit InventoryManager(std::shared_ptr<ThreadPool> thread_pool);
+
+    void setInventory(const ProductId& productId, ProductCount quantity);
+};