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 parent's `__isset()`
and `__get()` methods.

Related to #6065
Merge pull request #6120 from thenbrent/protected_variation_meta_data

Allow WC_Product_Variation children to extend meta