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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sqs_client = aws_stack.connect_to_service('sqs')
    queues = sqs_client.list_queues()
    url = name if '://' in name else aws_stack.get_sqs_queue_url(name)
    for queue_url in queues['QueueUrls']:
        if queue_url == url:
            return True
def topic_exists(name):
    sns_client = aws_stack.connect_to_service('sns')
    topics = sns_client.list_topics()
    for topic in topics['Topics']:
        topic_arn = topic['TopicArn']
        if topic_arn.endswith(':%s' % name):
            return topic_arn
def queue_url_exists(queue_url):
    sqs_client = aws_stack.connect_to_service('sqs')
    queues = sqs_client.list_queues()
    return queue_url in queues['QueueUrls']
def stream_exists(name):
    kinesis_client = aws_stack.connect_to_service('kinesis')
    streams = kinesis_client.list_streams()
    return name in streams['StreamNames']
def get_stack_details(stack_name):
    cloudformation = aws_stack.connect_to_service('cloudformation')
    stacks = cloudformation.describe_stacks(StackName=stack_name)
    for stack in stacks['Stacks']:
        if stack['StackName'] == stack_name:
            return stack
def describe_stack_resource(stack_name, resource_logical_id):