Include ConsumerGroupConsumers field in file.Content

[randmonkey]

In dbless mode of kong gateway, caller need to configure relationship between consumers and consumer groups in consumer_group_consumers, like:

{
  "_format_version": "3.0",
  "consumers": [
    {
      "username": "test-consumer"
    }
  ],
  "consumer_group_consumers": [
    {
      "consumer_group": "test-consumer-group-1",
      "consumer": "test-consumer"
    }
  ],
  "consumer_groups": [
    {
      "name": "test-consumer-group-1"
    }
  ]
}

but now file.Content does not contain such a field, so we cannot configure it to Kong running in dbless mode via decK. This would block Kong/kubernetes-ingress-controller#4448.