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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cloudformation = aws_stack.connect_to_service('cloudformation')
        invalid_json = '{"this is invalid JSON"="bobbins"}'
        try:
            cloudformation.validate_template(TemplateBody=invalid_json)
            self.fail('Should raise ValidationError')
        except (ClientError, ResponseParserError) as err:
            if isinstance(err, ClientError):
                self.assertEqual(err.response['ResponseMetadata']['HTTPStatusCode'], 400)
                self.assertEqual(err.response['Error']['Message'], 'Template Validation Error')
    def test_list_stack_resources_returns_queue_urls(self):
        cloudformation = aws_stack.connect_to_resource('cloudformation')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_2))
        cloudformation.create_stack(StackName=TEST_STACK_NAME_2, TemplateBody=template)
        def check_stack():
            stack = get_stack_details(TEST_STACK_NAME_2)
            self.assertEqual(stack['StackStatus'], 'CREATE_COMPLETE')
        retry(check_stack, retries=3, sleep=2)
        stack_summaries = list_stack_resources(TEST_STACK_NAME_2)
        queue_urls = get_queue_urls()
        topic_arns = get_topic_arns()
        stack_queues = [r for r in stack_summaries if r['ResourceType'] == 'AWS::SQS::Queue']
        for resource in stack_queues:
            self.assertIn(resource['PhysicalResourceId'], queue_urls)
        stack_topics = [r for r in stack_summaries if r['ResourceType'] == 'AWS::SNS::Topic']
        for resource in stack_topics:
            self.assertIn(resource['PhysicalResourceId'], topic_arns)