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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try:
        queue_arn = aws_stack.sqs_queue_arn(queue_name, region_name=region_name)
        sources = get_event_sources(source_arn=queue_arn)
        LOG.debug('Found %s source mappings for event from SQS queue %s' % (len(sources), queue_arn))
        source = next(iter(sources), None)
        if source:
            arn = source['FunctionArn']
            event = {'Records': [{
                'body': message_body,
                'receiptHandle': 'MessageReceiptHandle',
                'md5OfBody': md5(message_body),
                'eventSourceARN': queue_arn,
                'eventSource': 'aws:sqs',
                'awsRegion': region_name,
                'messageId': str(uuid.uuid4()),
                'attributes': {
                    'ApproximateFirstReceiveTimestamp': '{}000'.format(int(time.time())),
                    'SenderId': TEST_AWS_ACCOUNT_ID,
                    'ApproximateReceiveCount': '1',
                    'SentTimestamp': '{}000'.format(int(time.time()))
                },
                'messageAttributes': message_attributes,
                'sqs': True,
            }]}
            run_lambda(event=event, context={}, func_arn=arn)
            return True
    except Exception as e:
        LOG.warning('Unable to run Lambda function on SQS messages: %s %s' % (e, traceback.format_exc()))
def get_event_sources(func_name=None, source_arn=None):