Merge bitcoindevkit/bdk#645: Allow signing only specific leaf hashes

Merge bitcoindevkit/bdk#645: Allow signing only specific leaf hashes

a713a5a0629c9a643708a4b0d8d6ac3a87a13195 Better customize signing in taproot transactions (Daniela Brozzoni)

Pull request description:

  Fixes #616

  ### 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
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK a713a5a0629c9a643708a4b0d8d6ac3a87a13195

Tree-SHA512: 1100d43cb394b429450fc34f49dd815a024701987c0e6dd163865bd5c4c6f7102127b1ea6e10ced5fdb319874be97baeeb0deea66b4138410871a1d68b4def10