Expose the add_foreign_utxo method on TxBuilder type

[thunderbiscuit]

Description

This PR fixes #329.

Notes to the reviewers

Not ready for review yet.

Changelog notice

APIs Added:
    - Add TxBuilder.add_foreign_utxo method [#358]

[#358]: https://github.com/bitcoindevkit/bdk-ffi/pull/358

• I've signed all my commits
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• 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

[BitcoinZavior]

I have tested and it works well inside Flutter.
This will help close LtbLightning/bdk-flutter#86

[thunderbiscuit]

I had previously added the PartiallySignedTransaction.inputs() method to this PR, thinking it was needed, but after review I see that it's not required the for the feature we're building here.

I could add it back if we think there is demand for it, but for now removing it seems like the simplest solution.

[mightyguava]

Hello, we have a use case for this. We need to sweep/drain funds from multiple wallets to a single destination address. The mobile app has the public descriptors for all wallets. I'm hoping to do this with a single transaction. It seemed like this method would be the one to use to combine psbts together but it's not exposed via ffi yet.

[thunderbiscuit]

Closing this as additional features will not be merged in the 0.X version of bdk-ffi. The new 1.0 API will not require this anymore (or at least not in the same form), so further work on this is not necessary. All efforts are currently going to the 1.0 release! 🚀