Improve the UX of various bundle settings

When selecting an order type for an order item type, or a product variation type for a product type, we always show a select box.

We need to modify the code so that if there's up to 7 options, we use radios instead of a select.

Question: Should we hide the setting if there's only one option and the field is required? It looks clean, but might confuse users when they come back the next time and see a new element (due to additional options being available).

This is very close to the commerce_entity_select behavior, so maybe we can at least steal some tricks from there (or modify & reuse that element).