### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

SNS allows for the publishing of messages to an SMS destination with a phone number. Those messages are logged and stored in the `SnsStore`, but there is no way to retrieve them for verification. The request here is to expose a new endpoint which returns SMS messages from the store, similar to the one made available to retrieve email messages sent via SES.

### 🧑‍💻 Implementation

The `SNSServicePlatformEndpointMessagesApiResource` class makes endpoints available for accessing platform endpoint messages sent via SNS. A similar resource class could be implemented to provide new endpoints for accessing SMS messages.

Given that interaction platform endpoint messages is exposed at the path `/_aws/sns/platform-endpoint-messages`, SMS message interaction would presumably be exposed at `/_aws/sns/sms-messages`. A `GET` endpoint could accept an optional `phoneNumber` query parameter to filter messages based on the destination phone number.

```python
class SNSServiceSMSMessagesApiResource:
  @route(sns_constants.SMS_MSGS_ENDPOINT, methods=["GET"])
    def on_get(self, request: Request):
      account_id = #...
      region = #...
      filter_phone_number = request.args.get("phoneNumber")
      # get store; conditionally filter messages based on filter_phone_number
```

### Anything else?

_No response_
self,
        sns_create_topic,
        sqs_create_queue,
        sqs_queue_arn,
        sns_subscription,
        snapshot,
        aws_client,
    ):
        topic_arn = sns_create_topic()["TopicArn"]
        queue_url = sqs_create_queue()
        queue_arn = sqs_queue_arn(queue_url)
        subscription = sns_subscription(TopicArn=topic_arn, Protocol="sqs", Endpoint=queue_arn)
        snapshot.match("sub", subscription)
        subscription_arn = subscription["SubscriptionArn"]
        response = aws_client.sns.get_subscription_attributes(SubscriptionArn=subscription_arn)
        subscription_attributes = response["Attributes"]
        snapshot.match("sub-attrs", response)
        assert subscription_attributes["SubscriptionArn"] == subscription_arn
        subscriptions_by_topic = aws_client.sns.list_subscriptions_by_topic(TopicArn=topic_arn)
        snapshot.match("subscriptions-for-topic-before-unsub", subscriptions_by_topic)
        assert len(subscriptions_by_topic["Subscriptions"]) == 1
        aws_client.sns.unsubscribe(SubscriptionArn=subscription_arn)
        def check_subscription_deleted():