Make `core.CasmClass` `ByteCodeSegmentLenghts` pointer field

[EgeCaner]

Seems like not all Casm Classes has ByteCodeSegmentLenghts field, rust type definition uses Option type. This difference in representation was causing inconsistencies while go-rust communication, while rust was expecting Option::None, we were providing Zero Values. To have a workaround in Juno v0.15.11 we changed the starknet.CasmClass.ByteCodeSegmentLenghts to pointer field, we use this type while both fetching the class from FGW and in rust <-> go communication. Considering this change, we should also update the core.CasmClass to use pointer field