Add Support for ProductGroup, hasVariant, variesBy, and productGroupID

Problem/Motivation

Google recently added support for the existing product variant scheme, as referenced here: https://www.seroundtable.com/google-product-variant-structured-data-3692...

Steps to reproduce

Existing product schema is fantastic, but this new recognition from Google should encourage an expansion of the Product Schema for the Metatag module.

Proposed resolution

Following best practices as outlined by Google, make the new fields available within Product Schema for them to be added through product and variant entities.