Hitbtc3 fetchFundingRate
Hitbtc3 fetchFundingRate

Added fetchFundingRate to Hitbtc3:
```
hitbtc3.fetchFundingRate (BTC/USDT:USDT)
2022-03-22T08:49:06.669Z iteration 0 passed in 198 ms

{
  info: {
    contract_type: 'perpetual',
    mark_price: '42657.35',
    index_price: '42653.52',
    funding_rate: '0.0001',
    open_interest: '30.9861',
    next_funding_time: '2022-03-22T16:00:00.000Z',
    indicative_funding_rate: '0.0001',
    premium_index: '0',
    avg_premium_index: '0.000029587712038098',
    interest_rate: '0.0001',
    timestamp: '2022-03-22T08:49:05.685Z'
  },
  symbol: 'BTC/USDT:USDT',
  markPrice: 42657.35,
  indexPrice: 42653.52,
  interestRate: 0.0001,
  estimatedSettlePrice: undefined,
  timestamp: 1647938945685,
  datetime: '2022-03-22T08:49:05.685Z',
  fundingRate: 0.0001,
  fundingTimestamp: undefined,
  fundingDatetime: undefined,
  nextFundingRate: 0.0001,
  nextFundingTimestamp: 1647964800000,
  nextFundingDatetime: '2022-03-22T16:00:00.000Z',
  previousFundingRate: undefined,
  previousFundingTimestamp: undefined,
  previousFundingDatetime: undefined
}
```

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