Make it possible to construct proposals that spend stored transparent funds (P2PKH and P2SH)

[str4d]

We don't necessarily want this behaviour exposed in e.g. the mobile SDKs, but it is necessary functionality for a zcashd wallet replacement, and needs to be implemented somewhere (maybe behind a feature flag).

Currently this functionality exists in zcashd_client_backend but is restricted to wallet-internal shielding transactions (which logically means we do not support spending from stored transparent funds).