* feat: support new alternative_estimate_fee option - mempoolspacemedian
* chore: introduce a fallback median fee rate in case it comes zero from mempool space response
* chore: add and test util function for rounding to significant digits
* chore: use rounding of medianFee to 3 significant digits
* chore: make the new alternative_estimate_fee be configurable, change its name from Median to Block
* chore: return 1000 sats/kB fee for MempoolSpaceBlock estimation method when block not identified
* chore: make the fallback fee rate configurable, improve tests, improve function names
