Make it clear that CWallet::GenerateNewZKey is Sprout specific
Closes https://github.com/zcash/zcash/issues/3577
When adding sapling support we had considered making the method `GenerateNewZKey` generic, but it ended up making more sense to add a second method `GenerateNewSaplingZKey` for sapling support.
This PR changes the name of `GenerateNewZKey` to `GenerateNewSproutZKey` and changes the return type from `libzcash::PaymentAddress` to `libzcash::SproutPaymentAddress` to make it more clear that this is what the method does.