Merge bitcoindevkit/bdk#728: Add fee_amount() and fee_rate() functions to PsbtUtils...

ab41679368be79e8645915b40be554cc07557931 Add fee_amount() and fee_rate() functions to PsbtUtils trait (Steve Myers)

Pull request description:

  ### Description

  The purpose of the PR is to provide a more convenient way to calculate the transaction fee amount and fee rate for a PSBT. This PR adds `fee_amount` and `fee_rate` functions to the existing `PsbtUtils` trait and implements them for `PartiallySignedBitcoinTransaction`. The `fee_rate` value is only valid if the PSBT it is called on is fully signed and finalized.

  See related discussion: https://github.com/bitcoindevkit/bdk-ffi/issues/179

  ### Changelog

  Added
  - PsbtUtils.fee_amount(), calculates the PSBT total transaction fee amount in Sats.
  - PsbtUtils.fee_rate(), calculates the PSBT FeeRate, the value is only accurate AFTER the PSBT is finalized.

  ### Notes to the reviewers

  Ideally I'd like `fee_rate` to return an `Option` and return `None` if the PSBT isn't finalized. But I'm not quite sure how to determine if a PSBT is finalized without having a `Wallet` and running it through the finalize code first. Or there might be a way to fill in missing signatures with properly sized fake data prior to calculating the fee rate. For now I think it's enough to do this simple approach with usage warning in the rust docs.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  afilini:
    ACK ab41679368be79e8645915b40be554cc07557931

Tree-SHA512: 5386109c9ffcf63160f18b4c51eb2c582f4121b669c7276aaba489d186cf9b97343d46f887469b1407ccd7a24448b5e7aee4c6832d86768239b21f4e68054a0f