feat: expose only_witness_utxo and add_foreign_utxo on TxBuilder

[kumulynja]

Description

This PR adds the only_witness_utxo and add_foreign_utxo on TxBuilder since they are available in bdk, but missing in bdk-ffi. They are useful for bdk bindings to be used together with for example bark (Ark implementation from Second Tech) so a unilateral exit can be done with Cpfp transactions.

Notes to the reviewers

Since the add_foreign_utxo function is still experimental, I added the same comments mentioning this and the caution that has to be taken. I think it is good to expose it already though for experimentation like with bark as mentioned before.

Documentation

• bdk-ffi

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 a changelog in the next release tracking issue (see example)

New Features:

• I've added tests for the new feature
• I've added docs for the new feature