How to specify ParentId of existing "AWS::ApiGateway::RestApi" for "AWS::ApiGateway::Resource" in CloudFormation template? For example, I have already created API Gateway REST API and I want to specify it in my template:

```
 MyTestResource:
    Type: 'AWS::ApiGateway::Resource'
    Properties:
      RestApiId:
        Ref: 'RestApi'
      ParentId: <<placeholder_for_my_value>>

```

In case when I describe "AWS::ApiGateway::RestApi" in my template too I can do that such way:

```
  ParentId: !GetAtt "RestApi.RootResourceId"

```

But how can I do that for already existing REST API?