Method WriteCallback

src/quip_wallet.cpp:699–703

private:
  static size_t WriteCallback(void *contents, size_t size, size_t nmemb,
                              std::string *userp) {
    userp->append((char *)contents, size * nmemb);
    return size * nmemb;
  }

  nlohmann::json sendJsonRpc(const std::string &method,
                             const nlohmann::json &params) const {