Also use librustzcash for signature hashes (sighash) in V4 transactions and earlier

[conradoplg]

Is your feature request related to a problem? Please describe.

Zebra has its own signature hash (sighash) implementation for V4 transactions and earlier. For V5, it will use librustzcash (#2051) . Therefore, we'll have two different code paths.

Describe the solution you'd like

Also use librustzcash for V4 and earlier.

Describe alternatives you've considered

We could do nothing.

Additional context

The downside of this idea is additional overhead of reserializing and deserializing with librustzcash. However, we already accepted that for V5 transactions, and it could be improved in this issue or in a later one (by e.g. converting to librustzcash while deserializing, and not when computing the signature hash, so it will only happen once).

#2051 already adds a test for comparing librustzcash sighashes with zebra sighashes, and it passes (with a small bug fix in librustzcash that is being discussed with its devs). Therefore we already have some assurance that this change won't break anything.