message: fix compiler error for CameraState <-> JSON
  needed to be forward declared for createNodeFromJSON to see that
  CameraState was an avaialable type to convert to/from
diff: // default nlohmann::json will sort map alphabetically. this leaves it as-is
  using JSON = nlohmann::ordered_json;
  +inline void to_json(JSON &j, const CameraState &cs);
  +inline void from_json(const JSON &j, CameraState &cs);
  +
  // rkcommon type declarations /////////////////////////////////////////
  namespace rkcommon {
  