Specify fee rate for loop in

[C-Otto]

When I issue a loop in swap, I'd like to have more control about the fee rate that is used for the swap. I'm aware of --external, but I'd prefer a built-in solution that is an alternative to --conf_target.

Describe the solution you'd like
A new parameter --fee_rate (in sat/vByte) that is used as an alternative to --conf_target.

Describe alternatives you've considered
--external