【C++】：允许用户在类中添加额外内容

类似IParameter，添加模板类型转换as，这可以让用户自行添加额外内容。

class VFLOW_ABI IParameter {
public:
  virtual ~IParameter() = default;

  NO_DISCARD virtual std::shared_ptr<IParameter> clone() const = 0;

  NO_DISCARD virtual Buffer to_json() const = 0;
  NO_DISCARD virtual Buffer dump() const = 0;

  virtual IParameter &from_json(const Buffer &buf) = 0;
  virtual IParameter &load(const Buffer &buf) = 0;

  template <typename ParameterType>
  inline const ParameterType &as() const {
    static_assert(std::is_base_of_v<IParameter, ParameterType>,
                  "ParameterType must be a subtype of IParameter");
    return dynamic_cast<const ParameterType &>(*this);
  }

  template <typename ParameterType>
  inline ParameterType &as() {
    static_assert(std::is_base_of_v<IParameter, ParameterType>,
                  "ParameterType must be a subtype of IParameter");
    return dynamic_cast<ParameterType &>(*this);
  }

private:
  friend cereal::access;
  template <typename Archive>
  void serialize(Archive &ar, uint32_t version) {}
};