Add support for fee estimation with sub 1 satoshi

Currently, it is possible to broadcast transactions with a fee rate of sub-satoshi (below 1 sat/vB). However, the Esplora API is not capable of estimating fees with sub-1 satoshi. Even for 1008 blocks as a target, it still returns as 1:

```js
{
'1': 3.15,
'2': 3.15,
'3': 2.2520000000000002,
'4': 2.132,
'5': 2.132,
'6': 2.132,
'7': 1.199,
'8': 1.199,
'9': 1.199,
'10': 1.199,
'11': 1.199,
'12': 1.199,
'13': 1.081,
'14': 1.081,
'15': 1.081,
'16': 1.015,
'17': 1.015,
'18': 1,
'19': 1,
'20': 1,
'21': 1,
'22': 1,
'23': 1,
'24': 1,
'25': 1,
'144': 1,
'504': 1,
'1008': 1
}
```

It would be good to add support for estimations as low as `0.1` sat / vB.