### 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_
},
        }
        # p
        aws_client.sns.publish(
            TopicArn=topic_arn,
            Message=message_for_topic_string,
            MessageAttributes=message_attributes,
            MessageStructure="json",
        )
        # p
        aws_client.sns.publish(
            TargetArn=endpoint_arn_2,
            Message=message,
            MessageAttributes=message_attributes,
            MessageStructure="json",
        )
        # a
        def check_message():
            assert len(sns_backend.platform_endpoint_messages[endpoint_arn]) > 0
        retry(check_message, retries=PUBLICATION_RETRIES, sleep=PUBLICATION_TIMEOUT)
        msgs_url = config.get_edge_url() + PLATFORM_ENDPOINT_MSGS_ENDPOINT
        api_contents = requests.get(msgs_url).json()
        api_platform_endpoints_msgs = api_contents["platform_endpoint_messages"]
        assert len(api_platform_endpoints_msgs) == 2
        assert len(api_platform_endpoints_msgs[endpoint_arn]) == 1
        assert len(api_platform_endpoints_msgs[endpoint_arn_2]) == 1
        assert api_contents["region"] == "us-east-1"
        assert api_platform_endpoints_msgs[endpoint_arn][0]["Message"] == json.dumps(
            message_for_topic["APNS"]
        )