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 ref == 'AWS::Region':
        return aws_stack.get_region()
    resource_status = {}
    if stack_name:
        resource_status = describe_stack_resource(stack_name, ref)
        attr_value = resource_status.get(attribute)
        if attr_value not in [None, '']:
            return attr_value
    elif ref in resources:
        resource_status = resources[ref]['__details__']
    
    resource = resources.get(ref)
    resource_new = retrieve_resource_details(ref, resource_status, resources, stack_name)
    if not resource_new:
        return
    resource_type = get_resource_type(resource)
    result = extract_resource_attribute(resource_type, resource_new, attribute)
    if not result:
        LOG.warning('Unable to extract reference attribute %s from resource: %s' % (attribute, resource_new))
    return result
def resolve_refs_recursively(stack_name, value, resources):