refactor(wallet)!: make internal descriptor optional in constructors

[notmandatory]

Description

WIP! needs more testing to ensure everything works without a change descriptor.

If OK then can add new constructors for single descriptor wallets in a post 1.0 release.

Built on #1512 and should fix #1511.

Notes to the reviewers

Changelog notice

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

[storopoli]

Nice! I think it is a good compromise for now.

By the way, this still checks if the descriptors are unique right?
What we're actually doing is making the internal optional, but the checks that we've did in #1390 still apply, right?

[notmandatory]

By the way, this still checks if the descriptors are unique right? What we're actually doing is making the internal optional, but the checks that we've did in #1390 still apply, right?

Yes I kept the check that prevents duplicate descriptors, I just don't add a second descriptor and use the external one for change addresses. So for example if the user asked for addresses or utxos from the internal keychain they'll always get None. Correction this would break the API since we don't currently return an Option when requesting addresses 🙁. This could be a bigger problem.

To keep the current API the same (not add Option for returned AddressInfo) we'd need to either 1. allow it to panic if requesting address for non-existent internal keychain, 2. if internal doesn't exist pull address from external keychain instead. In ether case would have to explain careful what could happen.

[notmandatory]

closing in favor of #1533