Retrieve per-step stats

{  "object": "list",  "data": [    {      "object": "sequence_step_stats",      "step_id": "string",      "type": "delay",      "current": 0,      "entered": 0,      "completed": 0,      "sent": 0,      "skipped": 0,      "failed": 0,      "exited": 0,      "branch_true": 0,      "branch_false": 0,      "event_received": 0,      "timed_out": 0,      "opened": 0,      "clicked": 0    }  ]}