获取设备票据中参数需传SN   这个SN是怎么获取的
POST https://api.weixin.qq.com/wxa/getsnticket?access_token=ACCESS_TOKEN 

wx.requestSubscribeDeviceMessage({
    sn: 'xxxx',
    snTicket: 'xxxxx',
    modelId: 'xxxxx',
    tmplIds: ['xxxxx'],
    success(res) {
        console.log('[wx.requestSubscribeDeviceMessage success]: ', res)
        // { 'QCpBsp1TGJ1ML-UIwAIMkdXpPGzxSfwJqsKsvMVs3io': 'accept' }
    },
    fail(res) {
        console.log('[wx.requestSubscribeDeviceMessage fail]: ', res)
    }
})