With the recent tests added for serialiser search option in test driver and gRPC changes accordingly, its expected from performer to handle the case ie implement a custom serialiser and pass it to serialisersearch option and return result fields as deserialised via fieldAs.

Implementation expectation is well explained as comment in gRPC changes.

FIT change: https://review.couchbase.org/c/transactions-fit-performer/+/214787
Java performer implementation: https://review.couchbase.org/c/couchbase-jvm-clients/+/214786