Add an availability checker which checks if the entity is accessible or not

Re-rolled the patch from #19 to the latest 2.x-dev, because it failed to apply on commerce_order.services.yml.

@jsacksick shouldn't this target 3.0.x then based on #34 and get finished to be merged into that before release? (Just read through the comments and you wrote "Well, this is a needed functionality IMO")

I'll leave changing the target version to you.

This is a valid addition. However, with 3.x already released, we missed the opportunity to introduce it in a new major version—especially since it might not be backward compatible. That said, this is actually a good thing. We should ideally avoid introducing backward-incompatible changes even between major versions, when possible.

I believe this new checker can still be added, as long as it only affects new sites and existing sites retain their current behavior.

To make it easier for our core audience—ambitious site builders—to enable or disable availability rules and checkers as needed, I’ve proposed ✨ Expose Availability Checkers in the Admin UI Active .