Add missing Wallet APIs related to revealing and querying the wallet for addresses and indices

[thunderbiscuit]

This PR adds a bunch of the little methods we found last week were missing from the Wallet APIs. They're all mostly one-liners so it was easy to tackle.

Changelog notice

Added:
  - Wallet::peek_address method [#599]
  - Wallet::next_derivation_index [#599]
  - Wallet::next_unused_address [#599]
  - Wallet::mark_used [#599]
  - Wallet::reveal_addresses_to [#599]
  - Wallet::list_unused_addresses [#599]

[#599]: https://github.com/bitcoindevkit/bdk-ffi/pull/599

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

[rustaceanrob]

LGTM

[reez]

ACK a58599f

looking forward to using list_unused_addresses that’s a nice one, thanks for adding all of these