Get one campaign including its messages.

{  "campaign_id": "example",  "name": "Customer documents",  "status": "ACTIVE",  "archived": false,  "audiences": {    "included": [],    "excluded": []  },  "send_strategy": {    "method": "GET",    "datetime": "example",    "date": "example",    "throttle_percentage": 0,    "is_local": false,    "send_past_recipients_immediately": false  },  "use_smart_sending": false,  "tracking_options": {    "add_tracking_params": false,    "custom_tracking_params": [],    "is_tracking_opens": false,    "is_tracking_clicks": false  },  "created_at": "example",  "scheduled_at": "example",  "updated_at": "example",  "send_time": "example",  "campaign_message_ids": [],  "tag_ids": [],  "messages": []}