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!
except Exception as e:
        LOG.warning('Error calling %s with params: %s for resource: %s' % (function, params, resource))
        raise e
    
    if resource_type == 'ApiGateway::Method':
        integration = resource_props.get('Integration')
        if integration:
            api_id = resolve_refs_recursively(stack_name, resource_props['RestApiId'], resources)
            res_id = resolve_refs_recursively(stack_name, resource_props['ResourceId'], resources)
            uri = integration.get('Uri')
            if uri:
                uri = resolve_refs_recursively(stack_name, uri, resources)
                aws_stack.connect_to_service('apigateway').put_integration(restApiId=api_id, resourceId=res_id,
                    httpMethod=resource_props['HttpMethod'], type=integration['Type'],
                    integrationHttpMethod=integration['IntegrationHttpMethod'], uri=uri
                )
    elif resource_type == 'SNS::Topic':
        subscriptions = resource_props.get('Subscription', [])
        for subscription in subscriptions:
            endpoint = resolve_refs_recursively(stack_name, subscription['Endpoint'], resources)
            topic_arn = retrieve_topic_arn(params['Name'])
            aws_stack.connect_to_service('sns').subscribe(
                TopicArn=topic_arn, Protocol=subscription['Protocol'], Endpoint=endpoint)
    elif resource_type == 'S3::Bucket':
        tags = resource_props.get('Tags')
        if tags:
            aws_stack.connect_to_service('s3').put_bucket_tagging(
                Bucket=params['Bucket'], Tagging={'TagSet': tags})
    return result
def deploy_template(template, stack_name):