Enable converting the unauthorized tx output of the post-refactor tx builder into a PCZT

[str4d]

Currently the transaction builder produces proofs and signatures in-line. Once #1624 is completed, it will be possible to separate these concerns within Rust code on the same machine. This issue is about extending the separation of concerns by enabling the unauthorized (or partially-authorized) Rust types to be encoded as PCZTs for conveyance between machines.