### 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_
MessageStructure="json",
        )
        snapshot.match("duplicate-json-keys", resp)
        with pytest.raises(ClientError) as e:
            message = json.dumps({"default": {"object": "test"}})
            aws_client.sns.publish(
                TopicArn=topic_arn,
                Message=message,
                MessageStructure="json",
            )
        snapshot.match("key-is-not-string", e.value.response)
    @pytest.mark.aws_validated
    def test_message_structure_json_to_sqs(
        self, aws_client, sns_create_topic, sqs_create_queue, snapshot, sns_create_s
    ):
        topic_arn = sns_create_topic()["TopicArn"]
        queue_name = f"test-queue-{short_uid()}"
        queue_url = sqs_create_queue(QueueName=queue_name)
        sns_create_s(topic_arn=topic_arn, queue_url=queue_url)
        message = json.dumps({"default": "default field", "sqs": json.dumps({"field": "value"})})
        aws_client.sns.publish(
            TopicArn=topic_arn,
            Message=message,
            MessageStructure="json",
        )
        response = aws_client.sqs.receive_message(
            QueueUrl=queue_url, WaitTimeSeconds=10, MaxNumberOfMessages=1
        )
        snapshot.match("get-msg-json-sqs", response)
        receipt_handle = response["Messages"][0]["ReceiptHandle"]