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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pattern = 'arn:aws:states:%s:%s:stateMachine:%s'
    return _resource_arn(name, pattern, account_id=account_id, region_name=region_name)
def stepfunctions_activity_arn(name, account_id=None, region_name=None):
    pattern = 'arn:aws:states:%s:%s:activity:%s'
    return _resource_arn(name, pattern, account_id=account_id, region_name=region_name)
def fix_arn(arn):
    """ Function that attempts to "canonicalize" the given ARN. This includes converting
        resource names to ARNs, replacing incorrect regions, account IDs, etc. """
    if arn.startswith('arn:aws:lambda'):
        return lambda_function_arn(lambda_function_name(arn))
    LOG.warning('Unable to fix/canonicalize ARN: %s' % arn)
    return arn
def cognito_user_pool_arn(user_pool_id, account_id=None, region_name=None):
    pattern = 'arn:aws:cognito-idp:%s:%s:userpool/%s'
    return _resource_arn(user_pool_id, pattern, account_id=account_id, region_name=region_name)
def kinesis_stream_arn(stream_name, account_id=None):
    account_id = get_account_id(account_id)
    return 'arn:aws:kinesis:%s:%s:stream/%s' % (get_region(), account_id, stream_name)
def firehose_stream_arn(stream_name, account_id=None):