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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""" Create new event source mapping
        ---
        operationId: 'createEventSourceMapping'
        parameters:
            - name: 'request'
              in: body
    """
    data = json.loads(to_str(request.data))
    mapping = add_event_source(data['FunctionName'], data['EventSourceArn'], data.get('Enabled'))
    return jsonify(mapping)
@app.route('%s/event-source-mappings/<mapping_uuid>' % PATH_ROOT, methods=['PUT'])
def update_event_source_mapping(mapping_uuid):
    """ Update an existing event source mapping
        ---
        operationId: 'updateEventSourceMapping'
        parameters:
            - name: 'request'
              in: body
    """
    data = json.loads(request.data)
    if not mapping_uuid:
        return jsonify({})
    function_name = data.get('FunctionName') or ''
    enabled = data.get('Enabled', True)
    batch_size = data.get('BatchSize') or 100
    mapping = update_event_source(mapping_uuid, function_name, enabled, batch_size)
    return jsonify(mapping)
@app.route('%s/event-source-mappings/<mapping_uuid>' % PATH_ROOT, methods=['DELETE'])
def delete_event_source_mapping(mapping_uuid):