获取余额
请求地址：
https://sslv2.1688916.com/api/v1/finance/balance
Method：GET
请求头：
apiKey：用户apiKey
返回参数示例：
```
{
"code": 200,
"msg": "ok",
"data": {
"balance": 1000000
}
}
```
**返回码：**
200 => success.
400 => faild.
1000 => merchant dose not exist.
1001 => Order ID does not exist.
1002 => Order has already been cancelled.
1003 => Order can not be cancelled. Certificate has been issued for more than 7 days.
1004 => Insufficient account balance.