zcash_client_backend: Refactor create_proposed_transactions to support PCZTs

[str4d]

Currently create_proposed_transactions takes a TransactionProposal and synchronously produces all of its corresponding transactions, including proofs and signatures. We instead want to separate out the proving and signing steps by enabling (via a new method) returning the unauthorized transactions after their Builder::build step, so that the caller has the option of performing proving or signing on a different device.