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!
'MemorySize': 'MemorySize',
                
            },
            'defaults': {
                'Role': 'test_role'
            }
        }
    },
    'Lambda::Version': {
        'create': {
            'function': 'publish_version',
            'parameters': {
                'FunctionName': 'FunctionName',
                'CodeSha256': 'CodeSha256',
                'Description': 'Description'
            }
        }
    },
    'Lambda::Permission': {},
    'Lambda::EventSourceMapping': {
        'create': {
            'function': 'create_event_source_mapping',
            'parameters': {
                'FunctionName': 'FunctionName',
                'EventSourceArn': 'EventSourceArn',
                'StartingPosition': 'StartingPosition',
                'Enabled': 'Enabled',
                'BatchSize': 'BatchSize',
                'StartingPositionTimestamp': 'StartingPositionTimestamp'
            }