Backward Transfer P2PKH or P2SH

[Cermelli]

In the current implementation backward transfers are interpreted as P2PKH outputs. This implicitly causes a limit in terms of certificate outputs considering that current code limit the number of SigOpCounts per tx (see https://github.com/ZencashOfficial/zen/blob/merge_support_branch/src/main.cpp#L2755). A possible alternative would be to use P2SH as Backward transfers outputs that should not be considered in the SigOpCount calculation. Moreover this choice would save some (3?) bytes per output.