Return `FeeRate` type in TxDetail instead of `FeeRate::to_sat_per_vb_ceil()` (f32)

See [code that performs this here](https://github.com/bitcoindevkit/bdk-ffi/blob/a3d83653c39e5a29117d0fa6b4051d3971fabb6d/bdk-ffi/src/types.rs#L1327).

This currently means users cannot compute an accurate to any number of decimal points the FeeRate when returned by the `TxDetails` struct.