Make code loading carts using the cart provider easier to customize

Refactored in the MR:

• Drupal\commerce_cart\Controller\CartController
• Drupal\commerce_cart\Controller\CartBlock

Other places that could follow the same:

• Drupal\commerce_cart\Context\CartCacheContext
• Drupal\commerce_cart\EventSubscriber\QueryAccessSubscriber
• commerce_cart_user_login() - this might be easier to customize if moved to a hook service

2.x MRS aren't going to be merged, all MRS should be opened against 3.x.