server: allow router to report child instances sleep status (#20849)

* server: allow router to report child instances sleep status

* refactor

* move sleeping to state

* nits

tools/server/server-context.h

@@ -74,6 +74,10 @@ struct server_context {
     // get server metadata (read-only), can only be called after load_model()
     // not thread-safe, should only be used from the main thread
     server_context_meta get_meta() const;
+
+    // register a callback to be called when sleeping state changes
+    // must be set before load_model() is called
+    void on_sleeping_changed(std::function<void(bool)> callback);
 };