[WIP] Add merchant customer service module support

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>[Feature] 新增商家客服模块支持</issue_title>
<issue_description>## 功能描述
商家客服模块支持向用户发送客服消息，目前WxJava完全未实现。

官方文档

https://developers.weixin.qq.com/doc/channels/API/kf/

需要实现的接口列表（2个）

接口名称	请求路径
上传多媒体资源	/channels/ec/commkf/cosupload
发送消息	/channels/ec/commkf/sendmsg

实现要点

1. 新建服务接口

public interface WxChannelKfService {
  /**
   * 上传多媒体资源
   * @param type 资源类型: image/video/file
   * @param file 文件内容
   * @return media_id
   */
  String uploadMedia(String type, byte[] file) throws WxErrorException;
  
  /**
   * 发送客服消息
   * @param openId 用户openid
   * @param msgType 消息类型: text/image/video/file/product_share/order_share
   * @param content 消息内容
   */
  void sendMessage(String openId, String msgType, Object content) throws WxErrorException;
}

</issue_description>

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