Allow WC_Product_Variation children to extend meta

When SHA: 50a0577b81 introduced `$variation_level_meta_data` and `$variation_inherited_meta_data`, it set their access to `private`.

Child classes of `WC_Product_Variation` should be able to extend these properties to allow them to take advantage of the `__isset()` and __get()` methods and they don't have to duplicate the `WC_Product_Variation->variation_level_meta_data` and `WC_Product_Variation->variation_inherited_meta_data` values.

Related to #6065.