MGP - filter assignment objects for gb

fixes CNVS-17103

test plan:
 * turn on MGP feature flag
 * setup 2 assignments in the same assignment group but different
   grading periods
 * using the assignment_groups api #show endpoint
 - add the grading_period_id parameter with the appropriate id
   - &grading_period_id=<id>
 > it should only return one assignment

Change-Id: Ie10a509d8dee4980fd10d86627af8196279f654d
Reviewed-on: https://gerrit.instructure.com/45615
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>