feat: expose wallet signing with provided signers

[reez]

#1030 first

Description

Adds wrapper for bdk_wallet::Wallet::sign_with_signers, allowing bindings consumers to sign a PSBT with explicit SignersContainer values instead of only the wallet-owned signers.

This also adds a SignersContainer wrapper that can be constructed from descriptor key material and passed into the new wallet signing method.

Notes to the reviewers

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

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