Expose derivation_index method on wallet type

[thunderbiscuit]

Title says it all.

The method allows you to get the highest derivation index used for a given keychain.

Changelog notice

Added
    - Add `wallet.derivation_index` method on Wallet type [#579]

[#579]: https://github.com/bitcoindevkit/bdk-ffi/pull/579

Checklists

All Submissions:

• [ ] I've signed all my commits
• [ ] I followed the contribution guidelines
• [ ] I ran cargo fmt and cargo clippy before committing

New Features:

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

Bugfixes:

• [ ] This pull request breaks the existing API
• [ ] I've added tests to reproduce the issue which are now passing
• [ ] I'm linking the issue being fixed by this PR