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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""" Check whether a valid region is provided, and if not then raise an Exception. """
    auth_header = headers.get('Authorization')
    if not auth_header:
        raise Exception('Unable to find "Authorization" header in request')
    replaced = re.sub(r'.*Credential=([^,]+),.*', r'\1', auth_header)
    if auth_header == replaced:
        raise Exception('Unable to find "Credential" section in "Authorization" header')
    
    
    parts = replaced.split('/')
    region = parts[2]
    if region not in config.VALID_REGIONS:
        raise Exception('Invalid region specified in "Authorization" header: "%s"' % region)
def fix_account_id_in_arns(response, colon_delimiter=':', existing=None, replace=None):
    """ Fix the account ID in the ARNs returned in the given Flask response or string """
    existing = existing or ['123456789', MOTO_ACCOUNT_ID]
    existing = existing if isinstance(existing, list) else [existing]
    replace = replace or TEST_AWS_ACCOUNT_ID
    is_str_obj = is_string_or_bytes(response)
    content = to_str(response if is_str_obj else response._content)
    replace = r'arn{col}aws{col}\1{col}\2{col}{acc}{col}'.format(col=colon_delimiter, acc=replace)
    for acc_id in existing:
        regex = r'arn{col}aws{col}([^:%]+){col}([^:%]*){col}{acc}{col}'.format(col=colon_delimiter, acc=acc_id)
        content = re.sub(regex, replace, content)
    if not is_str_obj:
        response._content = content
        response.headers['content-length'] = len(response._content)
        return response
    return content
def get_s3_client():