[FR]: Implement `UtxoPerKbyte` for custom fees

[smk762]

Requested via Discord -

it's good to see that UtxoPerKbyte is any easily added implementation, a big upgrade, in my opinion.. allowing us to choose what fees we want to pay, depending on our price and time requirements..
as this value is always changing from block to block, i didn't feel equipped enough to know what fraction of btc to allocate... as you say, wouldn't want the tx to get stuck in the mempool for weeks... 
i feel i'd be more open to move funds in and out more freely, if that option was there. 
a nice addition i feel. 

[tonymorony]

There are also services similar to eth gas station, e.g. : https://mempool.space/docs/api/rest#get-recommended-fees (thank you @DeckerSU for the hint) which can be used for BTC fees better suggesting I noticed that sometimes I have to wait for 5+ blocks to get my btc transaction mined if it was sent with atomicdex

[cipig]

those extimates are per kb... the fee you need to pay depends on the size of the tx, which you can't know beforehand since it depends on how many utxos are used as input for your tx, which you can't know before the tx is sent/constructed specifying a fee per entire tx does not make sense at all without knowing it's size btw, the recommended fee can be easily seen on explorers, like https://blockchair.com/bitcoin

[smk762]

A solution for this should include:

• toggle to switch from fixed fee to fee per byte
• where available, suggested prices (ideally via api urls contained in coins repo)

[cipig]

toggle to switch from fixed fee to fee per byte

idk if we actually need the fixed fee... i personally would say no... so i would just replace fixed fee with fee per byte