FTX: fetchOpenInterest
FTX: fetchOpenInterest

```
node examples/js/cli ftx fetchOpenInterest BTC/USD:USD
```
```
ftx.fetchOpenInterest (BTC/USD:USD)
2022-09-30T22:01:42.436Z iteration 0 passed in 90 ms

{
  symbol: 'BTC/USD:USD',
  openInterestAmount: 64768.3668,
  openInterestValue: undefined,
  timestamp: undefined,
  datetime: undefined,
  info: {
    volume: '208126.4559',
    nextFundingRate: '-6e-6',
    nextFundingTime: '2022-09-30T23:00:00+00:00',
    openInterest: '64768.3668'
  }
}
2022-09-30T22:01:42.436Z iteration 1 passed in 90 ms
```

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