Implement BINARY_TYPE serialization

[raffclar]

Describe the issue

At these locations, support for BINARY_TYPE is missing
https://github.com/syoyo/tinygltf/blob/master/tiny_gltf.h#L6442
https://github.com/syoyo/tinygltf/blob/master/tiny_gltf.h#L6486

This case needs handling or tiny_gltf fails to output BINARY_TYPE values.

    case BINARY_TYPE:
      // TODO
      // obj = json(value.Get<std::vector<unsigned char>>());
      return false;
      break;

A workaround exists by using base64

// Serialize
std::map<std::string, tinygltf::Value> extras;
extras["custom"] = tinygltf::Value(tinygltf::base64_encode(data, length);
gltf.extras = tinygltf::Value(extras);
// Deserialize
tinygltf::base64_decode(gltf.extras.Get("custom").Get<std::string>());

To Reproduce

• OS: Any
• Compiler, compiler version, compile options: Any

tinygltf::Model gltf;
std::map<std::string, tinygltf::Value> extras;
auto someData = std::vector<unsigned char>({'a', 'b'});
extras["custom"] = tinygltf::Value(someData);
gltf.extras = tinygltf::Value(extras);

Expected behaviour

Handle BINARY_TYPE so that manual base64 encoding does not need to be done

[syoyo]

@raffclar I see. Having BINARY_TYPE support through base64 encoding would be nice. PR is much appreciated!