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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if not phys_res_id:
            if isinstance(resource, lambda_models.LambdaFunction):
                func_arn = aws_stack.lambda_function_arn(resource.function_name)
                resource.function_arn = resource.physical_resource_id = func_arn
            elif isinstance(resource, sfn_models.StateMachine):
                sm_arn = aws_stack.state_machine_arn(resource.name)
                resource.physical_resource_id = sm_arn
            elif isinstance(resource, service_models.StepFunctionsActivity):
                act_arn = aws_stack.stepfunctions_activity_arn(resource.params.get('Name'))
                resource.physical_resource_id = act_arn
            else:
                LOG.warning('Unable to determine physical_resource_id for resource %s' % type(resource))
    parse_and_create_resource_orig = parsing.parse_and_create_resource
    parsing.parse_and_create_resource = parse_and_create_resource
    parse_and_update_resource_orig = parsing.parse_and_update_resource
    parsing.parse_and_update_resource = parse_and_update_resource
    
    def parse_output(output_logical_id, output_json, resources_map):
        try:
            return parse_output_orig(output_logical_id, output_json, resources_map)
        except KeyError:
            output = Output()
            output.key = output_logical_id
            output.value = None
            output.description = output_json.get('Description')
            return output
    parse_output_orig = parsing.parse_output
    parsing.parse_output = parse_output
    
    def DynamoDB_Table_get_cfn_attribute(self, attribute_name):