Refactor Synchronizer.createProposedTransactions to expose PCZTs

[str4d]

Currently the public API takes a Proposal and returns the txids of fully-created and network-submitted transactions. We instead need to refactor this into several parts:

• An API that takes a Proposal and produces one or more unauthorized PCZTs.
• An API that takes a PCZT and applies whatever authorization steps the wallet is capable of doing (proof creation, signatures).
• An API that takes one or more authorized PCZTs and finalizes them, stores the resulting transactions in wallet storage, and submits the transactions to the network.

The existing API can then be a wrapper around these parts that continues to be a one-pass synchronous API for users that don't need to interact with other devices during transaction creation.