Func takes a FuncSpec and returns a *mapper.Func that can be called
to invoke this function. The callback can have an argument type of Args
in order to get access to the required dynamic proto.Any types of the
FuncSpec.
Spec takes a function pointer and generates a FuncSpec from it. The
function must only take arguments that are proto.Message implementations
or have a chain of converters that directly convert to a proto.Message.