mempool.
* WARNING: before version 0.21, fee_rate was in BTC/kvB. As of 0.21, fee_rate is in sat/vB. *

Arguments:
1. txid       (string, required) The txid to be bumped
2. options    (json object, optional) Options object that can be used to pass named arguments, listed below.

Named Arguments:
conf_target                    (numeric, optional, default=wallet -txconfirmtarget) Confirmation target in blocks
                               
fee_rate                       (numeric or string, optional, default=