feat: adapt `absurd_fee_per_kb` setting

Currently, absurd_fee_per_kb is hardcoded in Jam and used when validating the fee settings. absurd_fee_per_kb should be customizable by the user and it should be taken into account when validating fee values.

e.g. a user setting tx fee to 349 sats/vbyte with 20% randomization factor will likely have this tx failing without being properly informed exactly why (tx fee > absurd fee).