allow Option<T> as parameter types

[karroffel]

When a Rust method is exported to Godot, the type information of the parameters is used to convert the incoming Variants from Godot into the correct data type.

We could allow a new type, Option<T>, to be used as a parameter type as well. When trying to construct the value upon a method call it can be checked if the Variant is NULL or not, and construct the Option<T> accordingly.