Merge bitcoindevkit/bdk#562: Expose more getters in Wallet and other useful descripto...

1b9014846c5cb690f5c37f70556717b788a0733b Update changelog (Alekos Filini)
86abd8698f34d91f94bfce896affb535cca27146 [descriptor] Expose utilities to deal with derived descriptors (Alekos Filini)
0d9c2f76e00af801668fa2a70fcd13f9e399c0d2 [export] Use the new getters on `Wallet` to generate export JSONs (Alekos Filini)
63d5bcee934febe4d38622b677131bbe2e1a0abe [wallet] Add more getters (Alekos Filini)

Pull request description:

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->

  ### Description

  Expose more getters and internal utilities for people who need to work with descriptors.

  A good example of how this can be leveraged is in the second commit, which refactors the wallet export functionality to use the new public APIs rather than using members on `Wallet` directly.

  ### 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:

  * [ ] 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:
  notmandatory:
    ACK 8cd055090d75669c67a44467cc0281465ef31270

Tree-SHA512: 3e8833670ebc56316fce01fc572fcc9391d602ef85f0cde8edcb446295570a9012e18f6ba8af0984153e4688f66f7eea6803ef610ceb395867e464e05c01c137