feat(utxo-lib): add PSBT keyvalue helper methods for proprietary keys

Add standalone utility functions to handle proprietary keyvalues in PSBT, making
them usable even without access to the PSBT methods. These helpers support the
getting, adding, updating, and deleting of proprietary keyvalues from unknown
keyvalues arrays.

Issue: BTC-2652

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger