Should Funker functions be able to define types of args/return values?

This could probably be done with protobufs (like GRPC), but some questions that were raised:

- Does this add unnecessary complexity? Can we leave it up to the user to do type checking, if required?
- How are the types specified? Could they be exposed as labels on the image?

The issue names no files, tests, or entry points. Start by reviewing how Funker functions currently handle argument and return types, then evaluate protobuf/gRPC-style contracts, user-managed type checking, and exposing types as image labels; done means the project has an agreed approach to these questions.

Written by the indexing model from the issue text.