Add validation for the "type" of AssessmentItem objects #4877

Desired behavior

A choice option added to the AssessmentItem class type field to ensure that the types are limited to those that are acceptable and understood by Studio and Kolibri.

The choices can simply be set to the question_choices value set in le_utils: https://github.com/learningequality/le-utils/blob/main/le_utils/constants/exercises.py#L40

Current behavior

There is no validation on the type of AssessmentItem objects.