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!
result = SNS_Topic_get_cfn_attribute(self, attribute_name)
        if attribute_name.lower() in ['arn', 'topicarn']:
            result = aws_stack.fix_account_id_in_arns(result)
        return result
    IAM_Role_get_cfn_attribute_orig = iam_models.Role.get_cfn_attribute
    iam_models.Role.get_cfn_attribute = IAM_Role_get_cfn_attribute
    
    @classmethod
    def Lambda_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        resource_name = cloudformation_json.get('Properties', {}).get('FunctionName') or resource_name
        return Lambda_create_from_cloudformation_json_orig(resource_name, cloudformation_json, region_name)
    Lambda_create_from_cloudformation_json_orig = lambda_models.LambdaFunction.create_from_cloudformation_json
    lambda_models.LambdaFunction.create_from_cloudformation_json = Lambda_create_from_cloudformation_json
    
    @classmethod
    def Mapping_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json.get('Properties', {})
        func_name = props.get('FunctionName') or ''
        if ':lambda:' in func_name:
            props['FunctionName'] = aws_stack.lambda_function_name(func_name)
        return Mapping_create_from_cloudformation_json_orig(resource_name, cloudformation_json, region_name)
    Mapping_create_from_cloudformation_json_orig = lambda_models.EventSourceMapping.create_from_cloudformation_json
    lambda_models.EventSourceMapping.create_from_cloudformation_json = Mapping_create_from_cloudformation_json
    
    @classmethod
    def Lambda_update_from_cloudformation_json(cls,