Allow Extraction of PSBT struct

[BitcoinZavior]

Expose the entire PSBT in the bindings.
(https://docs.rs/bitcoin/0.29.2/bitcoin/util/psbt/struct.PartiallySignedTransaction.html)
This will allow the tx to be extracted with available data, verification of extracted tx.
This will also be useful and perhaps mandatory for multisig tx creation and signing.

Depends on bitcoindevkit/bdk-ffi#311