List subscribable events

Returns every event that an AiTask can subscribe to via trigger_type: "event" + event_conditions.event_type. Splits results into internal (always-on Crevio-emitted events like form_submission.created, order.paid) and external (this account's deployed event sources, e.g. external.gmail.new_email). The two task events, task_run.completed and task_run.failed, list required_conditions: ["after_task_id"]: a task subscribes to them only by naming the task it runs after.