Hi, I'm trying to get LocalStack set up for integration testing and having a lot of trouble building CloudFormation templates that include nested stacks referenced via the `TemplateURL` property. Is this just not a thing that works in LocalStack? I've tried setting TemplateURL to a local file as well as to an object in an S3 bucket created with LocalStack.

For example if I call `create-stack` on a `template.yaml` containing the following:
```
Resources:
  SNSTopics:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: http://localhost:4572/artifacts/sns.yaml
```

...the stack builds but the referenced `sns.yaml` stack does not get built; whereas if I call `create-stack` directly on the referenced file it builds fine.

Thanks for any help/advice you can give!
""" Find ID of the given resource. """
            if not resource:
                return
            for id_attr in ('Id', 'id', 'ResourceId', 'RestApiId', 'DeploymentId'):
                if id_attr in resource:
                    return resource[id_attr]
        
        if hasattr(resource, 'id') or (isinstance(resource, dict) and resource.get('id')):
            existing_id = resource.id if hasattr(resource, 'id') else resource['id']
            new_res_id = find_id(result)
            LOG.debug('Updating resource id: %s - %s, %s - %s' % (existing_id, new_res_id, resource, resource_json))
            if new_res_id:
                LOG.info('Updating resource ID from %s to %s (%s)' % (existing_id, new_res_id, region_name))
                update_resource_id(resource, new_res_id, props, region_name)
            else:
                LOG.warning('Unable to extract id for resource %s: %s' % (logical_id, result))
        
        update_physical_resource_id(resource)
        return resource
    def update_resource_name(resource, resource_json):
        """ Some resources require minor fixes in their CF resource definition
            before we can pass them on to deployment. """
        props = resource_json['Properties'] = resource_json.get('Properties') or {}
        if isinstance(resource, sfn_models.StateMachine) and not props.get('StateMachineName'):
            props['StateMachineName'] = resource.name
    def update_resource_id(resource, new_id, props, region_name):