Reverse Swaps: Allow for custom claim network fee

[kilrau]

As per https://github.com/BoltzExchange/boltz-backend/issues/326#issuecomment-1651683088 we want to allow for users to chose a custom sat/vbyte claim network fee via e.g. a slider.

Currently web app estimates fee before calling /createswap in order to be able to give exact receive amounts. The custom fee slider should clearly show how the receive amount changes when bumpgin or lower the fee.