### 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_
"Timestamp": timestamp_millis(),
    }
    
    if not subscriber.get("TopicArn", "").endswith(".fifo"):
        data.update(
            {
                "SignatureVersion": "1",
                
                
                
                "Signature": "EXAMPLEpH+..",
                "SigningCertURL": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-0000000000000000000000.pem",
            }
        )
    else:
        data["SequenceNumber"] = message_context.sequencer_number
    if message_type == "Notification":
        unsubscribe_url = create_unsubscribe_url(external_url, subscriber["SubscriptionArn"])
        data["UnsubscribeURL"] = unsubscribe_url
    elif message_type in ("UnsubscribeConfirmation", "SubscriptionConfirmation"):
        data["Token"] = message_context.token
        data["SubscribeURL"] = create_subscribe_url(
            external_url, subscriber["TopicArn"], message_context.token
        )
    if message_context.subject:
        data["Subject"] = message_context.subject
    if message_context.message_attributes:
        data["MessageAttributes"] = prepare_message_attributes(message_context.message_attributes)
    return json.dumps(data)
def prepare_message_attributes(