curl --request GET \
  --url https://realtime-api.voiceflow.com/v1/stable/function-variable \
  --header 'Authorization: Bearer <token>'

{
  "variables": [
    {
      "id": "<string>",
      "name": "<string>",
      "createdAt": "2023-11-07T05:31:56Z",
      "updatedAt": "2023-11-07T05:31:56Z",
      "functionID": "<string>",
      "description": "<string>"
    }
  ]
}