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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return 1000 * 60
def cleanup():
    global event_source_mappings, arn_to_lambda
    arn_to_lambda = {}
    event_source_mappings = []
    LAMBDA_EXECUTOR.cleanup()
def func_arn(function_name):
    return aws_stack.lambda_function_arn(function_name)
def add_function_mapping(lambda_name, lambda_handler, lambda_cwd=None):
    arn = func_arn(lambda_name)
    arn_to_lambda[arn].versions.get('$LATEST')['Function'] = lambda_handler
    arn_to_lambda[arn].cwd = lambda_cwd
def add_event_source(function_name, source_arn, enabled):
    mapping = {
        'UUID': str(uuid.uuid4()),
        'StateTransitionReason': 'User action',
        'LastModified': float(time.mktime(datetime.utcnow().timetuple())),
        'BatchSize': 100,
        'State': 'Enabled' if enabled is True or enabled is None else 'Disabled',
        'FunctionArn': func_arn(function_name),
        'EventSourceArn': source_arn,
        'LastProcessingResult': 'OK',
        'StartingPosition': LAMBDA_DEFAULT_STARTING_POSITION
    }
    event_source_mappings.append(mapping)
    return mapping
def update_event_source(uuid_value, function_name, enabled, batch_size):