[Woo] Extract subscriptions outside of WCProductModel table

[hichamboushaba]

⚠️ Please don't merge yet, this has some breaking changes.

This PR has two main changes:

• It removes the METADATA column from the WCProductModel table.
• It adds the ability to update the metadata separately when adding or updating a product.

Testing

Testing steps are provided in https://github.com/woocommerce/woocommerce-android/pull/12766 and https://github.com/woocommerce/woocommerce-android/pull/12767