添加微信客服SDK (#436)

* 添加微信客服SDK

* polish:优化签名函数

* polish：优化注释内容

* polish:复用已有的Token以及CommonError,移除无用的输出

* polish:复用已有的消息加解密

* fix:修复错误信息被覆盖的问题

* polish:go fmt 文件

work/work.go

@@ -4,6 +4,7 @@ import (
 	"github.com/silenceper/wechat/v2/credential"
 	"github.com/silenceper/wechat/v2/work/config"
 	"github.com/silenceper/wechat/v2/work/context"
+	"github.com/silenceper/wechat/v2/work/kf"
 	"github.com/silenceper/wechat/v2/work/msgaudit"
 	"github.com/silenceper/wechat/v2/work/oauth"
 )
@@ -37,3 +38,8 @@ func (wk *Work) GetOauth() *oauth.Oauth {
 func (wk *Work) GetMsgAudit() (*msgaudit.Client, error) {
 	return msgaudit.NewClient(wk.ctx.Config)
 }
+
+// GetKF get kf
+func (wk *Work) GetKF() (*kf.Client, error) {
+	return kf.NewClient(wk.ctx.Config)
+}