QuizAssignmentOverrideSet needs a container

The return signature of "Retrieve assignment-overridden dates for
quizzes" in the Quiz Assignment Overrides API returns its
QuizAssignmentOverrideSet objects inside another (currently)
anonymous object with a key of "quiz_assignment_overrides". This
de-anonymizes the object and names it
QuizAssignmentOverrideSetContainer. Necessary for machines to read
the API and know what to actually return.

Test Plan:
- A new QuizAssignmentOverrideSetContainer should exist in the API
  docs.
- The type returned by "Retrieve assignment-overridden dates for
  quizzes" should be of that type.

Fixes SIS-653

Change-Id: I9a1ce38824422760a2b20ceaca1722a64b9cf061
Reviewed-on: https://gerrit.instructure.com/44286
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>