Allow availability results to return a "code"

Describe your bug or feature request.

Availability checkers can return availability result objects specifying a "reason" for why a given order item is unavailable.

I'm suggesting we allow returning a "code" as well.
The code could be an integer, or a "string", don't really want to be super strict about it...

For reference, also created a core issue to ensure violation codes are returned by JSON API (See #3274805: Update the UnprocessableHttpEntityExceptionNormalizer to support returning violation codes).

Also changed the "reason" param type to "mixed", since we accept strings and markup interfaces...