**Describe the enhancement** <!-- A clear and concise description of what you would like added or changed. --> Use new FeeRate type in TxBuilder per [comment](https://github.com/bitcoindevkit/bdk-ffi/pull/437#discussion_r1434450464) in #437 "For another PR, but now that we have the FeeRate type, let's use it in the TxBuilder the same way it's used in Rust bdk." **Use case** <!-- Tell us how you or others will use this new feature or change to an existing feature. --> **Additional context** <!-- Add any other context about the enhancement here. -->
