Handle for an asynchronously started child workflow.
start_child/3 returns a %Continuum.ChildRef{}; pass it to await_child/1
to suspend until the child terminates and recover its result. The ref is
deterministic across replays:
child_run_id— the child run's id, derived deterministically from the parent run id, the start command id, and anyid:option, so the same parent at the same cursor never starts two children on replay.start_command_id— the command identity of thestart_childcall site.workflow— the child workflow module.