refactor: reduce certificate django apps dependence on the modulestore (generation_handler)

[MICROBA-1178]
- remove modulestore usage in `generation_handler.py`
- add duplicate functions that utilize a CourseKey or CourseOverview to remove dependence on modulestore (this will be cleaned up (if possible) at a later part of this refactor)
- add python API function to `content`/`course_overview` app that will retrieve a single CourseOverview (rather than a serialized list of dicts of CourseOverview data)

openedx/core/djangoapps/content/course_overviews/api.py

@@ -2,12 +2,18 @@
 CourseOverview internal api
 """
 from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
-
 from openedx.core.djangoapps.content.course_overviews.serializers import (
     CourseOverviewBaseSerializer,
 )
 
 
+def get_course_overview(course_id):
+    """
+    Retrieve and return course overview data for the provided course id.
+    """
+    return CourseOverview.get_from_id(course_id)
+
+
 def get_course_overviews(course_ids):
     """
     Return course_overview data for a given list of opaque_key course_ids.

openedx/core/djangoapps/content/course_overviews/tests/test_api.py

@@ -3,7 +3,7 @@ course_overview api tests
 """
 from django.test import TestCase
 
-from openedx.core.djangoapps.content.course_overviews.api import get_course_overviews
+from openedx.core.djangoapps.content.course_overviews.api import get_course_overview, get_course_overviews
 from openedx.core.djangoapps.content.course_overviews.tests.factories import CourseOverviewFactory
 
 from ..models import CourseOverview
@@ -19,6 +19,14 @@ class TestCourseOverviewsApi(TestCase):
         for _ in range(3):
             CourseOverviewFactory.create()
 
+    def test_get_course_overview(self):
+        """
+        Test for `get_course_overview` function to retrieve a single course overview.
+        """
+        course_overview = CourseOverviewFactory.create()
+        retrieved_course_overview = get_course_overview(course_overview.id)
+        assert course_overview.id == retrieved_course_overview.id
+
     def test_get_course_overviews(self):
         """
         get_course_overviews should return the expected CourseOverview data