Function get_existing_users_exercise_slide_for_course

pub async fn get_existing_users_exercise_slide_for_course(
    conn: &mut PgConnection,
    user_id: Uuid,
    exercise_id: Uuid,
    course_id: Uuid,
    fetch_service_info: impl Fn(Url) -> BoxFuture<'static, ModelResult<ExerciseServiceInfoApi>>,
) -> ModelResult<Option<CourseMaterialExerciseSlide>>