Add a new ProductViewBuilder to manage the product variation rendering

Problem/Motivation

Currently, the product variation fields are added to the Product render array in commerce_product_commerce_product_view() function.

When someone needs to rewrite that logic, it's necessary to implement that hook, or the _alter() version to override that behavior. That also forces to redo that logic again, hitting performance.

That's why we think a good improvement would be to add a service where that task in performed. In this way, this logic can be replaced without need to redo all the processing when users need to rewrite the default behavior.

Proposed resolution

Add the concept of ProductVariationRenderer, as tagged service where implement the logic explained above
Create a ProductVariationRendererManager service that takes care of managing the different Product Variation Renderers
Create a default service ProductVariationRendererDefault that implements the code the currently is in commerce_product_commerce_product_view()

Remaining tasks

User interface changes

None

API changes

Add a new service to manage Product Variation rendering inside products

Data model changes