Support the handling of objects with types that require custom logic to implement idiomatically in each language.

Examples:

• support returning objects of type std::optional<T> as nullable objects in the C API, and as the appropriate reference-or-null type in each language (e.g. None in Python).
• support std::vector<T> by exposing ptr, size in the C API and as appropriate types in each language (e.g. numpy.array in Python).
• support std::variant<A, B, C, ...> ...