zcash_client_backend: Make Sapling parameters optional for extract_and_store_transaction

[str4d]

We added the ability to check whether a PCZT contains a Sapling bundle, in order to determine whether Sapling parameters were necessary. But we forgot to make this part optional:

librustzcash/zcash_client_backend/src/data_api/wallet.rs

Lines 1856 to 1859 in 90d186f

	let transaction = TransactionExtractor::new(finalized)
	.with_sapling(spend_vk, output_vk)
	.with_orchard(orchard_vk)
	.extract()?;

Alter this API to let the Sapling parameters be optional (and we may as well also make Orchard optional).

[daira]

Oops :-). Btw is that a 400-line function?

[str4d]

Closed by #1729.