`zcash_client_backend`: Add support for full P2PKH history tracking

[str4d]

At present, we only track what is in the UTXO set at the time that UTXO information is requested from the light wallet server. This is sufficient for shielding funds, but insufficient for correct transparent history recovery or spending stored funds.

This issue is only about expanding support for the kinds of inputs we can currently spend, namely P2PKH. We will likely need additional information / tracking to be able to correctly track and spend P2SH outputs.