http请求方式：POST
https://api.weixin.qq.com/product/stock/get?access_token=xxxxxxxxx

{
    "product_id": 23423523452345235,
    "out_product_id": "1234566",
    "sku_id": 1343,
    "out_sku_id": "1024",
}

{
    "errcode": 0,
    "errmsg":"ok",
    "data": 
    {
       "stock_num": 100
    }
}