Async: Add a new Callback class for accessing caller's contin

We previously passed a sh_ptr to the caller's contin as arguments
to the std::bind() callable. In order for us to be able to trace
deadlocks, we need to be able to access them explicitly.

So here's that change.

include/callback.h

@@ -0,0 +1,39 @@
+#ifndef CALLBACK_H
+#define CALLBACK_H
+
+#include <memory>
+#include <functional>
+
+namespace smo {
+
+// Forward declaration
+class AsyncContinuation;
+
+/**
+ * @brief Callback class that wraps a function and its caller continuation
+ * 
+ * This class provides a way to pass both a callback function and the
+ * caller's continuation in a single object, enabling deadlock detection
+ * by walking the chain of continuations.
+ */
+template<typename CbFnT>
+class Callback
+{
+public:
+	/**
+	 * @brief Constructor
+	 * @param caller The caller's continuation
+	 * @param cb The callback function to invoke
+	 */
+	Callback(std::shared_ptr<AsyncContinuation> caller, std::function<CbFnT> cb)
+	: callerContinuation(std::move(caller)), callback(std::move(cb))
+	{}
+
+public:
+	std::shared_ptr<AsyncContinuation> callerContinuation;
+	std::function<CbFnT> callback;
+};
+
+} // namespace smo
+
+#endif // CALLBACK_H