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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response_obj = jsonify(response_obj)
        else:
            response_obj = str(response_obj)
            details['Headers']['Content-Type'] = 'text/plain'
        return response_obj, details['StatusCode'], details['Headers']
    
    not_found = None
    if arn not in arn_to_lambda:
        not_found = not_found_error(arn)
    elif qualifier and not arn_to_lambda.get(arn).qualifier_exists(qualifier):
        not_found = not_found_error('{0}:{1}'.format(arn, qualifier))
    if not_found:
        forward_result = forward_to_fallback_url(func_arn, data)
        if forward_result is not None:
            return _create_response(forward_result)
        return not_found
    if invocation_type == 'RequestResponse':
        result = run_lambda(asynchronous=False, func_arn=arn, event=data, context={}, version=qualifier)
        return _create_response(result)
    elif invocation_type == 'Event':
        run_lambda(asynchronous=True, func_arn=arn, event=data, context={}, version=qualifier)
        return _create_response('', status_code=202)
    elif invocation_type == 'DryRun':
        
        return _create_response('', status_code=204)
    return error_response('Invocation type not one of: RequestResponse, Event or DryRun',
                          code=400, error_type='InvalidParameterValueException')
@app.route('%s/event-source-mappings/' % PATH_ROOT, methods=['GET'])
def list_event_source_mappings():