Ability to distinguish between Transparent pending change and pending non-change

[pacu]

originated by #1411

function add_transparent_account_balances does not distinguish between change and non-change pending funds.

librustzcash/zcash_client_sqlite/src/wallet/transparent.rs

Lines 382 to 389 in bf8b39a

	#[tracing::instrument(skip(conn, account_balances))]
	pub(crate) fn add_transparent_account_balances(
	conn: &rusqlite::Connection,
	mempool_height: BlockHeight,
	account_balances: &mut HashMap<AccountId, AccountBalance>,
	) -> Result<(), SqliteClientError> {
	let mut stmt_account_balances = conn.prepare(
	"SELECT u.account_id, SUM(u.value_zat)

This responds to zcash_client_backend (backed by zcash_client_sqlite) not handling transparent change. When querying the AccountBalance of a particular account, the unshielded balance will return a Balance struct where the change_pending_confirmation field will always be NonNegativeAmount::ZERO.

Currently, zcash_client_backend assumes that ephemeral transparent addresses should only contain funds that will be spent on a subsequent transaction (e.g.: when sending funds from the shielded pool to a TEX Address).

librustzcash/zcash_client_backend/src/data_api/wallet.rs

Lines 1208 to 1223 in bf8b39a

	// We don't need the TransparentAddressMetadata here; we can look it up from the data source later.
	for ((change_index, change_value), (ephemeral_address, _)) in
	ephemeral_outputs.iter().zip(addresses_and_metadata)
	{
	// This output is ephemeral; we will report an error in `create_proposed_transactions`
	// if a later step does not consume it.
	builder.add_transparent_output(&ephemeral_address, change_value.value())?;
	transparent_output_meta.push((
	Recipient::EphemeralTransparent {
	receiving_account: account_id,
	ephemeral_address,
	outpoint_metadata: (),
	},
	ephemeral_address,
	change_value.value(),
	StepOutputIndex::Change(*change_index),

In order to enabling spending transparent funds beyond sending to TEX addresses, this assumption shouldn't be made anymore.

Related: #1360 #1370