Set messageAttribute in an SNS Notification when --notification-arns is populated

### Describe your idea/feature/enhancement

I am using the command below to deploy, and I want to be able to filter SNS notifications depending on the message attributes. However no attributes are provided, this requires me to make another lambda to just create message attributes then send that notification again.
```
sam deploy \
--template-file serverless-output.yaml \
--capabilities CAPABILITY_IAM \
--region $(REGION) \
--notification-arns $(NOTIFICATION_ARNS) \
--stack-name $(STACK_NAME)
```

### Proposal

Add details of how to add this to the product.

In order to make this easier, have the deploy function populate the messageAttributes field in the specificed sns notification ARNs.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)

### Additional Details