Fetch a recurring series by ext:/UUID reference

{  "object": "recurring_series",  "id": "string",  "external_id": "string",  "customer_id": "string",  "currency": "string",  "frequency": "monthly",  "start_date": "string",  "end_date": "string",  "issue_timing": {    "mode": "start-of-period"  },  "status": "draft",  "cancelled_at": "string",  "lines": [    {      "product_id": "string",      "recognition_method": "value",      "agreed_value": 0,      "hourly_rate": 0,      "discount": 0,      "description_override": "string",      "position": -9007199254740991,      "assignees": [        {          "user_id": "string",          "distribution_percentage": 0,          "hourly_rate_override": 0        }      ]    }  ]}