Allow for a dynamic control of the btc cash out fees

Right now, the btc trasnaction fee that pays to the the miners is set to 20sats/byte for all transactions. It would be cool if this could be controlled by the member who was doing the transaction. The best case scenario would be if it could be totally controlled, but the service would be very improved even if 3/4 options were given. For example, 20sat/byte, 14sat/byte, 7sat/byte and (1 or 2)sat/byte (the numbers are just an example).

PS: I think that the cash out fee could be adapted accordingly too - say if the transaction fee paid to the miners is x, then the cash out fee for a member is 10x, or 7x or whatever. But anyway, this should be discussed in an assembly. For now, I think that it would be good to just have in mind the improvement that I described. I would