Use FeeRate type in TxDetails fee_rate field

[thunderbiscuit]

Fixes #932.

The current implementation doesn't allow to calculate more precise versions of the feerate and always gives back the ceiling version of the feerate integer in sat/vb. This lets users do the calculations they need on the type.

It's too bad but this is actually a breaking change, so can only ship with 3.0.

Documentation

• bdk_wallet
  https://docs.rs/bdk_wallet/2.3.0/bdk_wallet/struct.TxDetails.html

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[thunderbiscuit]

cc @AlexBrou

[busayo-OD]

I’m new to the project and wanted to help test this change locally. I confirmed that TxDetails now uses the FeeRate type instead of an integer, which fixes the rounding issue on the Rust side.
For UniFFI consumers (Kotlin/Swift), I noticed that FeeRate currently exposes only integer-based accessors (e.g. to_sat_per_kwu() returning sat/kwu).
Do you expect consumers to perform the sat/kwu → sat/vB conversion themselves, or would it make sense to expose a helper that returns sat/vB as a float to make fractional fee rates directly accessible?