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!
stack_name = self._get_param('StackName')
        backend = self.cloudformation_backend
        stack = backend.get_stack(stack_name)
        if not stack:
            
            stack = ([stk for id, stk in backend.deleted_stacks.items() if stk.name == stack_name] or [0])[0]
        if not stack:
            raise ValidationError(stack_name,
                message='Unable to find stack "%s" in region %s' % (stack_name, aws_stack.get_region()))
        template = self.response_template(responses.DESCRIBE_STACK_EVENTS_RESPONSE)
        return template.render(stack=stack)
    responses.CloudFormationResponse.describe_stack_events = cf_describe_stack_events
def inject_stats_endpoint():
    """ Inject a simple /_stats endpoint into the moto server backend Web app. """
    
    from moto import server as moto_server
    def _get_stats():