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!
props = cloudformation_json['Properties']
        name = props['StageName']
        deployment_id = props.get('Id') or short_uid()
        description = props.get('Description') or ''
        return apigw_models.Deployment(deployment_id, name, description)
    @classmethod
    def Resource_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json['Properties']
        region_name = props.get('Region') or aws_stack.get_region()
        path_part = props.get('PathPart')
        api_id = props.get('RestApiId')
        parent_id = props.get('ParentId')
        id = props.get('Id') or short_uid()
        return apigw_models.Resource(id, region_name, api_id, path_part, parent_id)
    @classmethod
    def Method_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
        props = cloudformation_json['Properties']
        method_type = props.get('HttpMethod')
        authorization_type = props.get('AuthorizationType')
        return apigw_models.Method(method_type, authorization_type)
    apigw_models.RestAPI.create_from_cloudformation_json = RestAPI_create_from_cloudformation_json
    apigw_models.RestAPI.get_cfn_attribute = RestAPI_get_cfn_attribute
    apigw_models.Deployment.create_from_cloudformation_json = Deployment_create_from_cloudformation_json
    apigw_models.Resource.create_from_cloudformation_json = Resource_create_from_cloudformation_json
    apigw_models.Method.create_from_cloudformation_json = Method_create_from_cloudformation_json
    
    
    def describe_stack_resource(self):