Coinex setLeverage
Coinex setLeverage

Added setLeverage to Coinex:
```
node examples/js/cli coinex setLeverage 3 BTC/USDT:USDT
'{"position_type":"1"}'

coinex.setLeverage (3, BTC/USDT:USDT, [object Object])
2022-04-29T02:50:38.789Z iteration 0 passed in 291 ms

{
  code: '0',
  data: { leverage: '3', position_type: '1' },
  message: 'OK'
}
```

ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt