Add new _variations_skus meta field for products

[felipeelia]

Description of the Change

This PR adds a new meta field called _variations_skus to WooCommerce products with their variations SKUs. This is a revamp of #1828, with some changes:

1. Although perhaps not as performant, this PR relies more on code and WC solutions than a direct DB query
2. Instead of concatenating all SKUs in a single string, it actually creates an array of values
3. It does not add code to handle variations creations/edit/deletions because recent versions of WooCommerce will already trigger that update
4. It does not add code to change the admin search. That will come in WooCommerce behavior in the Product Admin List View #2757

Changelog Entry

Added: New _variations_skus field to WooCommerce products.

Credits

Props @felipeelia @kallehauge @lukaspawlik