Async: add sh_ptr<ContinuationChainLink> to Callback<>

This change enables us to finally implement the tracing of
continuations backward from the point of acquisition for deadlock
debugging.

include/asynchronousContinuationChainLink.h

@@ -0,0 +1,25 @@
+#ifndef ASYNCHRONOUS_CONTINUATION_CHAIN_LINK_H
+#define ASYNCHRONOUS_CONTINUATION_CHAIN_LINK_H
+
+#include <memory>
+
+namespace smo {
+
+/**
+ * @brief Base class for all asynchronous continuation chain links
+ *
+ * This non-template base class provides type erasure for the continuation
+ * chain, allowing RTTI and dynamic casting when walking the chain.
+ *
+ * The chain walking logic can use dynamic_cast to determine the most
+ * derived type and perform appropriate operations.
+ */
+class AsynchronousContinuationChainLink
+{
+public:
+    virtual ~AsynchronousContinuationChainLink() = default;
+};
+
+} // namespace smo
+
+#endif // ASYNCHRONOUS_CONTINUATION_CHAIN_LINK_H