[.NET] Use ObjectID when converting Variant to GodotObject

[raulsntos]

Use the ObjectID when converting Variant to GodotObject to avoid invalid Object pointers.

• Similar to GDExtension: Allow directly getting ObjectID from Variant #97119 and Directly get object instance ID from Variant and implement Variant::get_validated_object() godot-cpp#1591.
• Fixes C#: ChkCastAny InvalidCastException caused by freeing instances with references in Godot.Collections.Array<T> #92355.
• Supersedes [C#] Fix VariantUtils.ConvertTo<T> #92572.

[paulloz]

Looks good to me, and seems fine throughout my testing 👍

Now, the big discussion is about how we feel about returning null here. For me, it's kinda problematic in NRT context, when working with an Array<Foo> (not Array<Foo?>), and still potentially be given a null value 🤔 But I guess the only alternative is keeping a potential InvalidCastException, which does not feel particularly good either.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.4.1.