0.31.2 policy patch

[BitcoinZavior]

⚠️ Depends on bitcoindevkit/bdk-ffi#612

Fixes

• Resolved issue with building a transaction using a multi-sig descriptor resulted in a Spending policy error.
• Added support for specifying a policy path in tx_builder using policy_path, enabling correct multi-sig spending policy specification during transaction creation.

Enhancements

• Exposed policies() from the wallet to allow users to retrieve available spending policies for their wallet.