### 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_
ReturnSubscriptionArn=True,
        )
        message = json.dumps({"message": "hello world"})
        message_attributes = {
            "testAttribute": {"DataType": "String", "StringValue": "valueOfAttribute"}
        }
        aws_client.sns.publish(
            TopicArn=topic["TopicArn"], Message=message, MessageAttributes=message_attributes
        )
        def validate_content():
            files = aws_client.s3.list_objects(Bucket=bucket_name)["Contents"]
            f = BytesIO()
            aws_client.s3.download_fileobj(bucket_name, files[0]["Key"], f)
            content = to_str(f.getvalue())
            sns_message = json.loads(content.split("\n")[0])
            assert "Type" in sns_message
            assert "MessageId" in sns_message
            assert "Message" in sns_message
            assert "Timestamp" in sns_message
            assert message == sns_message["Message"]
        retries = 5
        sleep = 1
        sleep_before = 0
        if is_aws_cloud():
            retries = 30
            sleep = 10
            sleep_before = 10
        retry(validate_content, retries=retries, sleep_before=sleep_before, sleep=sleep)
    @pytest.mark.aws_validated
    def test_empty_or_wrong_message_attributes(