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 not isinstance(prop_keys, list):
                prop_keys = [prop_keys]
            for prop_key in prop_keys:
                if prop_key == PLACEHOLDER_RESOURCE_NAME:
                    params[param_key] = resource_id
                    resource_name = get_resource_name(resource)
                    if resource_name:
                        params[param_key] = resource_name
                    else:
                        
                        try:
                            return resolve_ref(stack_name, resource_id, resources,
                                attribute='PhysicalResourceId')
                        except Exception as e:
                            LOG.debug('Unable to extract physical id for resource %s: %s' % (resource_id, e))
                else:
                    if callable(prop_key):
                        prop_value = prop_key(resource_props, stack_name=stack_name, resources=resources)
                    else:
                        prop_value = resource_props.get(prop_key)
                    if prop_value is not None:
                        params[param_key] = prop_value
    
    for param_key, prop_keys in dict(params).items():
        tmp_value = params.get(param_key)
        if tmp_value is not None:
            params[param_key] = resolve_refs_recursively(stack_name, tmp_value, resources)
        
        if str(tmp_value).lower() in ['true', 'false']:
            params[param_key] = str(tmp_value).lower() == 'true'