add an async subscriber impl with thread pool

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

@@ -0,0 +1,55 @@
+#pragma once
+
+#include <atomic>
+#include <condition_variable>
+#include <functional>
+#include <mutex>
+#include <queue>
+#include <thread>
+#include <vector>
+
+class ThreadPool {
+   public:
+    explicit ThreadPool(
+        size_t num_threads = std::thread::hardware_concurrency());
+
+    ~ThreadPool() noexcept;
+
+    void enqueue(std::function<void()> task);
+
+    // Get the number of worker threads
+    [[nodiscard]] size_t thread_count() const noexcept {
+        return workers_.size();
+    }
+
+    // Wait for all queued tasks to complete
+    void wait_for_completion() noexcept;
+
+    // Disable copy and move
+    // sync primitives are not movable or copyable
+    ThreadPool(const ThreadPool&) = delete;
+    ThreadPool& operator=(const ThreadPool&) = delete;
+    ThreadPool(ThreadPool&&) = delete;
+    ThreadPool& operator=(ThreadPool&&) = delete;
+
+   private:
+    // Worker thread function
+    void worker();
+
+    // Thread management
+    std::vector<std::thread> workers_;
+
+    // Task queue
+    std::queue<std::function<void()>> tasks_;
+
+    // Synchronization primitives
+    std::mutex queue_mutex_;
+    std::condition_variable condition_;
+
+    // Shutdown flag
+    std::atomic<bool> stop_;
+
+    // Track active tasks for wait_for_completion
+    std::atomic<size_t> active_tasks_;
+    std::condition_variable completion_condition_;
+};