refactor message serializer to allow access to eg contacts

src/fragment_store/message_fragment_store.hpp

@@ -4,6 +4,8 @@
 #include "./fragment_store_i.hpp"
 #include "./fragment_store.hpp"
 
+#include "./message_serializer.hpp"
+
 #include <entt/entity/registry.hpp>
 #include <entt/container/dense_map.hpp>
 
@@ -36,10 +38,6 @@ namespace Fragment::Components {
 	};
 } // Fragment::Components
 
-struct MessageSerializerCallbacks : public SerializerCallbacks<Message3> {
-	// TODO: add contact and message reg, so entities can be looked up and be converted to fragment uids OR persistent ids
-};
-
 // handles fragments for messages
 // on new message: assign fuid
 // on new and update: mark as fragment dirty