feat(signer): merge internal and external signer containers

[ndungudedan]

Currently BDK stores signers in two separate SignersContainer instances - one for internal keychain and one for external. This separation is unnecessary since:

1. Most signers can sign for both keychains
2. Both containers are always called during signing
3. The separation adds complexity without benefits

This change:

• Adds an extend method to SignersContainer to support merging containers
• Keeps a single signer container in Wallet

Closes #186

Notes to the reviewers

• Looking for a concept ACK to validate that my approach is okay.
• 3 tests fail with Incompatible change descriptor, from the export module. On further investigation, I see that the export node returns the change descriptor as a public key while the derived change descriptor is a private key. I am not sure why this is case and a second review would be great:

change_descriptor: Some("tr([73c5da0a/86'/0'/0']tprv8fMn4hSKPRC1oaCPqxDb1JWtgkpeiQvZhsr8W2xuy3GEMkzoArcAWTfJxYb6Wj8XNNDWEjfYKK4wGQXh3ZUXhDF2NcnsALpWTeSwarJt7Vc/1/*)")

export.change_descriptor(): Some("tr([73c5da0a/86'/0'/0']tpubDC3pD7UZXnsgh3EBjbtBQiB1FnLask7UHBSunZ1DPK4dCFFZoFRkgxHB8gt42FvLzx1DpxfHWxAsYaY6b643RVcGjDxXxns7wKKYnnfEcbB/1/*)")

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo +nightly 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

[oleonardolima]

Thanks for working on this one, though, for the next breaking change release (v3.0), the plan is to remove the signers altogether from the Wallet API, see #70 and #235. That said, it supersedes the changes from this PR and issue #186, as it's also a breaking change.

[ValuedMammal]

Thanks for your contribution. I agree with @oleonardolima, assuming we go ahead with #70, then this issue probably won't be as relevant.

[ndungudedan]

Thanks! Closing the PR.