### 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_
subscription_arn = subscription["SubscriptionArn"]
            subscription_attrs = aws_client.sns.get_subscription_attributes(
                SubscriptionArn=subscription_arn
            )
            assert subscription_attrs["Attributes"]["PendingConfirmation"] == "false"
        retry(check_subscription, retries=PUBLICATION_RETRIES, sleep=PUBLICATION_TIMEOUT)
        aws_client.sns.publish(TopicArn=topic_arn, Subject=subject, Message=message)
        
        events = retry(
            check_expected_lambda_log_events_length,
            retries=10,
            sleep=1,
            function_name=function_name,
            expected_length=1,
            regex_filter="Records.*Sns",
            logs_client=aws_client.logs,
        )
        notification = events[0]["Records"][0]["Sns"]
        snapshot.match("notification", notification)
class TestSNSProvider:
    @pytest.mark.aws_validated
    def test_publish_unicode_chars(