List variants

curl --request GET \
  --url https://api.us.poly.ai/v1/agents/{agentId}/branches/{branchId}/variants \
  --header 'x-api-key: <api-key>'

{
  "variants": [
    {
      "id": "<string>",
      "name": "<string>",
      "isDefault": true,
      "attributeValues": {
        "values": {}
      }
    }
  ]
}