### 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_
)
        assert subscription_attributes["Attributes"]["PendingConfirmation"] == "false"
        message = "test_external_http_endpoint"
        aws_client.sns.publish(TopicArn=topic_arn, Message=message)
        assert poll_condition(
            lambda: len(server.log) >= 2,
            timeout=5,
        )
        notification_request, _ = server.log[1]
        assert notification_request.headers["x-amz-sns-message-type"] == "Notification"
        expected_unsubscribe_url = (
            f"{service_url}/?Action=Unsubscribe&SubscriptionArn={subscription_arn}"
        )
        if raw_message_delivery:
            payload = notification_request.data.decode()
            assert payload == message
        else:
            payload = notification_request.get_json(force=True)
            assert payload["Type"] == "Notification"
            assert "Signature" in payload
            assert "SigningCertURL" in payload
            assert payload["Message"] == message
            assert payload["UnsubscribeURL"] == expected_unsubscribe_url
            snapshot.match("http-message", payload)
        unsub_request = requests.get(expected_unsubscribe_url)
        unsubscribe_confirmation = xmltodict.parse(unsub_request.content)
        assert "UnsubscribeResponse" in unsubscribe_confirmation
        snapshot.match("unsubscribe-response", _get_snapshot_requests_response(unsub_request))
        assert poll_condition(
            lambda: len(server.log) >= 3,