Changes to GET `/api/v0/course-material/exercises/:exercise_id` - Get exercise by id. Includes relevant context so that doing the exercise is possible based on the response. This endpoint does not expose exercise's private spec because it would expose the correct answers to the user.

added the optional property current_exercise_slide/exercise_tasks/items/previous_submission/oneOf[#/components/schemas/ExerciseTaskSubmission]/data_files to the response with the 200 status

added the required property current_exercise_slide/exercise_tasks/items/previous_submission/oneOf[#/components/schemas/ExerciseTaskSubmission]/answer_kind to the response with the 200 status

added the optional property teacher_grading_decision to the response with the 200 status

added the new NotAnsweredAndLocked enum value to the exercise_status/oneOf[#/components/schemas/ExerciseStatus]/reviewing_stage response property for the response status 200