An XBlock runtime service for `can_view_answer`, ` can_view_correctness`?

I created an issue to track something that’s bugged me for a while, but I’m not sure how much of a problem it really is in practice. So to XBlock authors: would you use this if it was built?

The ProblemBlock has some sophisticated logic for determining whether the user should be allowed to see the answer (and also whether they were correct or not):

This has been partially re-implemented in other XBlocks over time. We should refactor this into a runtime service that will return a simple True/False for this question, so that we can get consistent behavior without other XBlock authors having to think about it.