Create a service for getting enabled product variations

Followup to #2506163: Complete the initial product architecture.

For the add to cart form we need a way to get all enabled variations of a product.

This means getting the variation ids referenced by the product, loading only the enabled ones (status = TRUE), then running an event on them (COMMERCE_PRODUCT.FILTER_VARIATIONS) to get the final list.

This should be implemented in a VariationManager (using the variation storage and the entity dispatcher as dependencies), with a getEnabledVariations($product) method. This will allow the logic to be easily swappable.