add an async subscriber impl with thread pool

subscriber-notifs-async/include/core/subscriber.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+template <typename TEventId, typename TEventInfo>
+class Subscriber {
+   public:
+    virtual void onNotification(const TEventId& eventId,
+                                const TEventInfo& eventInfo) = 0;
+
+    virtual ~Subscriber() = default;
+};