rpc: Design and implement `z_getbalances`

This is the RPC proposed in https://github.com/zcash/zcash/issues/6356 and drafted in https://github.com/zcash/zcash/pull/6389 that was never completed in `zcashd`. We should ensure it exists from the beginning in Zallet.

See also https://github.com/zcash/zcash/issues/6679.