将小程序获取ak的方式也抽象出来

miniprogram/context/context.go

@@ -1,18 +1,12 @@
 package context
 
 import (
-	"sync"
-
+	"github.com/silenceper/wechat/v2/credential"
 	"github.com/silenceper/wechat/v2/miniprogram/config"
 )
 
 // Context struct
 type Context struct {
 	*config.Config
-
-	//accessTokenLock 读写锁 同一个AppID一个
-	accessTokenLock *sync.RWMutex
-
-	//accessTokenFunc 自定义获取 access token 的方法
-	accessTokenFunc GetAccessTokenFunc
+	credential.AccessTokenHandle
 }