Bitget fetchLeverage
Bitget fetchLeverage

Added fetchLeverage to Bitget:
```
bitget.fetchLeverage (BTC/USDT:USDT)
2022-05-12T09:31:50.168Z iteration 0 passed in 275 ms

{
  code: '00000',
  msg: 'success',
  requestTime: '1652347910126',
  data: { symbol: 'BTCUSDT_UMCBL', minLeverage: '1', maxLeverage: '125' }
}
```

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