Expose `add_global_xpubs()` method on transaction builder

[thunderbiscuit]

This PR exposes the add_global_xpubs() method on the TxBuilder. See the docs here.

Fixes #572

Changelog notice

Added
  - Add `add_global_xpubs()` method on `TxBuilder` [#574]

[#574]: https://github.com/bitcoindevkit/bdk-ffi/pull/574

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] 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