Function headless_lms_models::pages::fetch_derived_spec

async fn fetch_derived_spec(
    existing_exercise_task: Option<&ExerciseTaskIdAndSpec>,
    task_update: &NormalizedCmsExerciseTask,
    urls_by_exercise_type: &HashMap<&String, Url>,
    spec_fetcher: impl SpecFetcher,
    previous_spec: Option<Value>,
    cms_block_id: Uuid
) -> Result<Option<Value>, ModelError>