Add missing PSBT and wallet RPC methods

[bc1cindy]

I'm contributing to the rust-payjoin(https://github.com/payjoin/rust-payjoin) project, and we recently replaced bitcoincore-rpc with a custom async client(payjoin/rust-payjoin#945) due to missing functionality in existing libraries.

The maintainers suggested contributing these missing methods upstream to benefit the entire Rust Bitcoin ecosystem. I'd like to help make bitcoind-async-client the go-to async Bitcoin RPC library.

Currently missing these essential methods for PSBT and wallet workflows:
PSBT Methods:

• wallet_create_funded_psbt - Creates funded PSBTs
• wallet_process_psbt - Signs PSBTs with wallet keys
• finalize_psbt - Finalizes PSBTs and extracts transactions

Utility Methods:

• get_address_info - Address validation and info
• list_unspent - Full UTXO filtering (current get_utxos has no parameters)

I have working implementations of all these methods from the payjoin-cli project that I can adapt to this library's architecture.

Would the maintainers be interested in these contributions? I'm happy to discuss the approach and submit PRs incrementally.

Related: payjoin/rust-payjoin#945