Add docs about IamportPayment and PaymentData

[SuhwanCha]

Overview
This PR writes document properties on IamportPayment and PaymentData.

Please follow link below to get more information about documentation:
https://dart.dev/guides/language/effective-dart/documentation

Usage

Todo

I can not found documentation about some properties,

  String? customerUid; // 정기결제 카드정보

  bool? popup; // 페이팔 팝업 여부
  String? naverUseCfm; // 네이버페이 이용완료일
  bool? naverPopupMode; // 네이버페이 팝업 여부
  List<NaverProducts>? naverProducts; // 네이버 상품정보
  bool? naverCultureBenefit;
  String? naverProductCode;
  String? naverActionType;
  NaverInterface? naverInterface;
  Map<String, String>? period; // [이니시스. 다날. 나이스] 서비스 제공기간 표기
  String? company; // [다날 - 휴대폰 소액결제 전용] 주문명: (company) name 대비
  bool? niceMobileV2 = true;

please document for this properties too.

[SuhwanCha]

Reference: https://docs.iamport.kr/sdk/javascript-sdk#request_pay

cc/ @anymate98

[SuhwanCha]

This file was not changed, but sync with upstream