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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from localstack.services.awslambda.lambda_api import BUCKET_MARKER_LOCAL
LOG = logging.getLogger(__name__)
CURRENTLY_UPDATING_RESOURCES = {}
RUN_SERVER_IN_PROCESS = False
MODEL_MAP = {
    'AWS::StepFunctions::Activity': service_models.StepFunctionsActivity
}
def start_cloudformation(port=None, asynchronous=False, update_listener=None):
    port = port or config.PORT_CLOUDFORMATION
    backend_port = DEFAULT_PORT_CLOUDFORMATION_BACKEND
    print('Starting mock CloudFormation (%s port %s)...' % (get_service_protocol(), port))
    start_proxy_for_service('cloudformation', port, backend_port, update_listener)
    if RUN_SERVER_IN_PROCESS:
        cmd = 'python "%s" cloudformation -p %s -H 0.0.0.0' % (__file__, backend_port)
        env_vars = {'PYTHONPATH': ':'.join(sys.path)}
        return do_run(cmd, asynchronous, env_vars=env_vars)
    else:
        argv = ['cloudformation', '-p', str(backend_port), '-H', '0.0.0.0']
        thread = FuncThread(start_up, argv)
        thread.start()
        return thread
def set_moto_account_ids(resource_json):
    def fix_ids(obj, **kwargs):
        if isinstance(obj, dict):
            for key, value in obj.items():
                if 'arn' in key.lower() and isinstance(value, six.string_types):
                    obj[key] = value.replace(TEST_AWS_ACCOUNT_ID, MOTO_ACCOUNT_ID)
        return obj
    return recurse_object(resource_json, fix_ids)
def get_entity_id(entity, resource_json=None):