Hi

I have issues creating sns subscriptions through Cloudformation. If I attempt to include a `AWS::SNS::Subscription` resource in a Cloudformation template and run `create-stack` then I get a 500 Internal Server error. The full template:

```
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Test'
Resources:
  MySnsTopic:
    Type: AWS::SNS::Topic
    Properties:
      TopicName: MySnsTopic
  MySnsTopicSubscription:
    Type: AWS::SNS::Subscription
    Properties:
      Protocol: sqs
      TopicArn: !Ref MySnsTopic
      Endpoint: !GetAtt
        - MySqsQueue
        - QueueArn
  MySqsQueue:
    Type: AWS::SQS::Queue
    Properties:
      QueueName: MySqsQueue
```

If I replace the references with hard-coded arns like this:

```
TopicArn: arn:aws:sns:eu-west-2:123456789012:MySnsTopic
Endpoint: arn:aws:sqs:elasticmq:000000000000:MySqsQueue
```

Then I get no errors but I get a warning:

`No moto Cloudformation support for AWS::SNS::Subscription`.

I can get around this issue by creating the subscription when creating the topic but the issue there is that I cannot add any attributes and in my case I need `RawMessageDelivery` set to true. So my questions are:

1. Is there no support to create `AWS::SNS::Subscription` resources through Cloudformation or am I doing something wrong?
2. If support is not there can I request it as a new feature?
3. Any suggestions on how I can work around this? I know I could do this through a script at startup but would like to not have any additional script steps other than deploying the Cloudformation stack.

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if hasattr(entity, 'physical_resource_id'):
        return entity.physical_resource_id
    
    types_with_ref_as_id_or_name = (apigw_models.RestAPI, apigw_models.Resource)
    attr_candidates = ['function_arn', 'Arn', 'id', 'name', 'Id', 'Name']
    for attr in attr_candidates:
        if hasattr(entity, attr):
            if attr in ['id', 'name'] and not isinstance(entity, types_with_ref_as_id_or_name):
                LOG.warning('Unable to find ARN, using "%s" instead: %s - %s',
                            attr, resource_json, entity)
            return getattr(entity, attr)
        if hasattr(entity, 'get_cfn_attribute'):
            try:
                result = entity.get_cfn_attribute(attr)
                if result:
                    return result
            except Exception:
                pass
    
    if hasattr(entity, 'params'):
        for key, value in (entity.params or {}).items():
            if key.endswith('Name'):
                return value
def convert_objs_to_ids(resource_json):