third-party JSON dependency included in header

[Zephilinox]

The third party dependency nlohmann/json is included in the header, which means clients of this SDK will also be pulling it in. this can conflict with a different but existing version of that library, and also means if a client is using a different JSON library, it now depends on multiple JSON libs. the path specified for that JSON dependency is also hardcoded

it would be great to move this dependency to the .cpp so clients don't need to worry about it