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!
'SNS::Topic': {
        'create': {
            'function': 'create_topic',
            'parameters': {
                'Name': 'TopicName',
                'Tags': 'Tags'
            }
        },
        'delete': {
            'function': 'delete_topic',
            'parameters': {
                'TopicArn': 'PhysicalResourceId'
            }
        }
    },
    'Logs::LogGroup': {
        
    },
    'Lambda::Function': {
        'create': {
            'function': 'create_function',
            'parameters': {
                'FunctionName': 'FunctionName',
                'Runtime': 'Runtime',
                'Role': 'Role',
                'Handler': 'Handler',
                'Code': get_lambda_code_param,
                'Description': 'Description',
                'Environment': 'Environment',
                'Timeout': 'Timeout',