code: public RefreshTokenResponse refreshToken(String grantType, String refreshToken) throws ApiException {
          ApiResponse<RefreshTokenResponse> resp = refreshTokenWithHttpInfo(grantType, refreshToken);
          return resp.getData();
      }
docs: Refresh Token
  Refresh Token
  @param grantType Grant Type. (required)
  @param refreshToken Refresh Token. (required)
  @return RefreshTokenResponse
  @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body