[inspector_protocol] Introduce protocol::Serializable

lib/FrontendChannel_h.template

Index: lib/FrontendChannel_h.template
diff --git a/lib/FrontendChannel_h.template b/lib/FrontendChannel_h.template
index 8b653b58214b80aaa2f288043aa45866e3346e45..0454978b0c8e88116cf89c66c0a4a9be6460f4f5 100644
--- a/lib/FrontendChannel_h.template
+++ b/lib/FrontendChannel_h.template
@@ -9,11 +9,17 @@
 namespace {{namespace}} {
 {% endfor %}
 
+class {{config.lib.export_macro}} Serializable {
+public:
+    virtual String serialize() = 0;
+    virtual ~Serializable() = default;
+};
+
 class {{config.lib.export_macro}} FrontendChannel {
 public:
     virtual ~FrontendChannel() { }
-    virtual void sendProtocolResponse(int callId, const String& message) = 0;
-    virtual void sendProtocolNotification(const String& message) = 0;
+    virtual void sendProtocolResponse(int callId, std::unique_ptr<Serializable> message) = 0;
+    virtual void sendProtocolNotification(std::unique_ptr<Serializable> message) = 0;
     virtual void flushProtocolNotifications() = 0;
 };