Custom type converters for method binding and variant casting

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With this API users can bind methods which have types in their signature that are not supported by default in the Godot C++ bindings. Closes godotengine/godot-proposals#14507 proposal.

Sample usage.

// Extracted from one of the headers of a library.

struct Vector {
  float x;
  float y;
  float z;
};

// In another source file. Define these converters so that `Vector` can be used in method signatures.

_FORCE_INLINE_ godot::Vector3 vector_to_vector3(const Vector &v) {
  return godot::Vector3(v.x, v.y, v.z);
}

_FORCE_INLINE_ Vector vector_from_vector3(const godot::Vector3 &v) {
  return Vector(v.x, v.y, v.z);
}

// This registers the converters so that they can be used in method bindings, variants, etc.
// We pass both `vector_to_vector3` and `vector_from_vector3` here.

GD_REGISTER_CONVERSION(Vector, godot::Vector3, vector_to_vector3, vector_from_vector3);

// In a source file which has a wrapper for a type from a library, in this case with the `Entity` class.
// Notice that we use the library types and that the converters are called elsewhere.

Vector Entity::get_position() const {
  return wrapped.position();
}

void Entity::set_position(const Vector &v) {
  wrapped.position() = v;
}

void Entity::_bind_methods() {
  godot::ClassDB::bind_method(godot::D_METHOD("get_position"), &Entity::get_position);
  godot::ClassDB::bind_method(godot::D_METHOD("set_position", "position"), &Entity::set_position);
}