ZB fetchFundingRate
ZB fetchFundingRate

Added fetchFundingRate to ZB:
```
zb.fetchFundingRate (BTC/USDT:USDT)
BTC_USDT
443 ms
{
  info: { fundingRate: '0.0001', nextCalculateTime: '2022-02-19 00:00:00' },
  symbol: 'BTC/USDT:USDT',
  markPrice: undefined,
  indexPrice: undefined,
  interestRate: undefined,
  estimatedSettlePrice: undefined,
  timestamp: undefined,
  datetime: undefined,
  fundingRate: 0.0001,
  fundingTimestamp: undefined,
  fundingDatetime: undefined,
  nextFundingRate: undefined,
  nextFundingTimestamp: 1645228800000,
  nextFundingDatetime: '2022-02-19 00:00:00',
  previousFundingRate: undefined,
  previousFundingTimestamp: undefined,
  previousFundingDatetime: undefined
}
```

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