Forward declaration for enum

[AF9036]

When working with json_fwd.h I don't see easy way to forward declare num types serialization.

I had to create 2 macros:

//this one used in header file

#define FORWARD_DECLARATION_NLOHMANN_ENUM(ENUM_TYPE)         \
  template <typename BasicJsonType>                          \
  void to_json(BasicJsonType& j, const ENUM_TYPE& e);        \
  template <typename BasicJsonType>                          \
  void from_json(const BasicJsonType& j, ENUM_TYPE& e);      \

//this one used in cpp file together with NLOHMANN_JSON_SERIALIZE_ENUM

#define INSTANTIATE_NLOHMANN_ENUM(ENUM_TYPE)                 \
template void to_json<nlohmann::json>(nlohmann::json& j, const ENUM_TYPE& e);   \
template void from_json<nlohmann::json>(const nlohmann::json& j, ENUM_TYPE& e); \

Would it be possible to add FORWARD_DECLARATION_NLOHMANN_ENUM into json_fwd.hpp ? Or is there any other simpler way to forward declare those num serialization functions ?