feat: Adds discussions settings for new discusions experience (#28749)

This commit adds new discussions settings for the new discussions experience. These are stored in the course so they can be a part of course import/export flow.
These are also added to the discussions configuraiton API to allow MFEs to update the settings.
The discussions API is currently available via LMS, however that means it cannot save changes to the modulestore. This also adds the API to the studio config so it can now also be accessed from studio and be used to save course settings.

common/lib/xmodule/xmodule/course_module.py

@@ -406,6 +406,16 @@ class CourseFields:  # lint-amnesty, pylint: disable=missing-class-docstring
             "If false, they are sorted chronologically by creation date and time."
         )
     )
+    discussions_settings = Dict(
+        display_name=_("Discussions Plugin Settings"),
+        scope=Scope.settings,
+        help=_("Settings for discussions plugins."),
+        default={
+            "enable_in_context": True,
+            "enable_graded_units": False,
+            "unit_level_visibility": False,
+        }
+    )
     announcement = Date(
         display_name=_("Course Announcement Date"),
         help=_("Enter the date to announce your course."),