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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sm_name = resolve_refs_recursively(stack_name, sm_name, resources)
            sfn_client = aws_stack.connect_to_service('stepfunctions')
            state_machines = sfn_client.list_state_machines()['stateMachines']
            sm_arn = [m['stateMachineArn'] for m in state_machines if m['name'] == sm_name]
            if not sm_arn:
                return None
            result = sfn_client.describe_state_machine(stateMachineArn=sm_arn[0])
            return result
        elif resource_type == 'StepFunctions::Activity':
            act_name = resource_props.get('Name') or resource_id
            act_name = resolve_refs_recursively(stack_name, act_name, resources)
            sfn_client = aws_stack.connect_to_service('stepfunctions')
            activities = sfn_client.list_activities()['activities']
            result = [a['activityArn'] for a in activities if a['name'] == act_name]
            if not result:
                return None
            return result[0]
        if is_deployable_resource(resource):
            LOG.warning('Unexpected resource type %s when resolving references of resource %s: %s' %
                        (resource_type, resource_id, resource))
    except Exception as e:
        check_not_found_exception(e, resource_type, resource, resource_status)
    return None
def check_not_found_exception(e, resource_type, resource, resource_status):
    
    markers = ['NoSuchBucket', 'ResourceNotFound', '404']
    if not list(filter(lambda marker, e=e: marker in str(e), markers)):
        LOG.warning('Unexpected error retrieving details for resource %s: %s %s - %s %s' %
            (resource_type, e, traceback.format_exc(), resource, resource_status))
def extract_resource_attribute(resource_type, resource, attribute):