Feature: Optional WIT-ification of function handler

[Gohlub]

It would be nice if we could specify that we don't want to include a specific message handler in WIT: If I am writing communication between FE and FE that involves complicated objects (like JSON Value type in Rust or a Hashmap e.g.), I'd much rather just exclude that definition from the WIT file to make it easier. Obviously, those really needing it to be in WIT can just avoid defining such handlers, but for our sake it would make writing/parsing arguments from the FE much more ergonomic.