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!
}
    },
    'DynamoDB::Table': {
        'create': {
            'function': 'create_table',
            'parameters': {
                'TableName': 'TableName',
                'AttributeDefinitions': 'AttributeDefinitions',
                'KeySchema': 'KeySchema',
                'ProvisionedThroughput': 'ProvisionedThroughput',
                'LocalSecondaryIndexes': 'LocalSecondaryIndexes',
                'GlobalSecondaryIndexes': 'GlobalSecondaryIndexes',
                'StreamSpecification': lambda params, **kwargs: (
                    common.merge_dicts(params.get('StreamSpecification'), {'StreamEnabled': True}, default=None))
            },
            'defaults': {
                'ProvisionedThroughput': {
                    'ReadCapacityUnits': 5,
                    'WriteCapacityUnits': 5
                }
            }
        }
    },
    'Events::Rule': {
        'create': [{
            'function': 'put_rule',
            'parameters': {
                'Name': PLACEHOLDER_RESOURCE_NAME,
                'ScheduleExpression': 'ScheduleExpression',
                'EventPattern': 'EventPattern',