Merge #1033: Backport new taproot descriptor template (BIP86)

7587f16 feat(descriptor): backport from master branch new taproot descriptor template (BIP86) (Steve Myers)
177c96d Create taproot descriptor template (Vladimir Fomene)

Pull request description:

  ### Description

  This PR solves #836 for the release/0.28 branch. This PR adds a P2TR descriptor template and a BIP86 taproot descriptor template. With this, users can now create a taproot descriptor with templates.

  ### Notes to the reviewers

  The commit from #840 is cherry-picked from the `master` branch to the `release/0.28` branch without any changes.

  ### Changelog notice

  Add taproot descriptor template (BIP-86).

  ### 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:
  danielabrozzoni:
    utACK 7587f16

Tree-SHA512: e5b07473e27bba8ca5ec58854fa318c5a82cb67ce751d352ef17e9926dd08f8e6b7a720a77170b6f6f018c58ed9f8741cee396dc8e8721f4022c33ef2904815f