Sandbox Mode for okex
Sandbox Mode for okex

I think in future we might unify this (together with 'setSandboxMode') as exchanges have different ways of 'set flag for sandbox mode'. at this moment, user has a 'hard-to-find' way to enable sandbox ( https://github.com/ccxt/ccxt/issues/7088 ) and I think CCXT might facilitate things. Maybe we can implement a unified param like:
```
const exch = new ccxt.xyz({
    ...
    'sandbox/demo': true
});

```

However, at this moment, for okex users might use this one too.
ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt