Add SKU to ProductBundleVariation for easier interoperability

Problem/Motivation

The shops we're working are using the variation SKUs for their internal systems. I'd request to define a new field like that or if that's not planned, then a getSku() method that would return 'BUNDLE_' . $this->id(); That way we would not have to rewrite all the external codes which are processing the carts and order items and expect there's some kind of SKU there (GA4 tracking, invoice generating, stock management).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes