Create a new agent session

{  "metadata": {},  "createdAt": "2019-08-24T14:15:22Z",  "activeStep": {    "reasoning": "string",    "tools": [      "string"    ],    "stepNumber": 0  },  "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",  "messages": [    {      "role": "user",      "createdAt": "2019-08-24T14:15:22Z",      "id": "string",      "content": [        {          "type": "text",          "text": "string"        }      ]    }  ],  "files": [    {      "name": "string",      "url": "string",      "type": "string"    }  ],  "actions": [    {      "createdAt": "2019-08-24T14:15:22Z",      "id": "string",      "type": "string",      "status": "pending",      "input": {},      "output": {}    }  ]}