Add support for fee estimation with sub 1 satoshi

[miguilimzero]

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:

{
  '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.