refactor: deduplicate MembershipWitness struct (backport #21244)#21344

AztecBot wants to merge 1 commit intobackport-to-v4-stagingAztecProtocol/aztec-packages:backport-to-v4-stagingfrom

claudebox/backport-21244-membership-witnessAztecProtocol/aztec-packages:claudebox/backport-21244-membership-witnessCopy head branch name to clipboard