### 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_
return susbcriber.get("RawMessageDelivery") in ("true", True, "True")
def validate_subscription_attribute(
    attribute_name: str, attribute_value: str, topic_arn: str
) -> None:
    """
    Validate the subscription attribute to be set. See:
    https://docs.aws.amazon.com/sns/latest/api/API_SetSubscriptionAttributes.html
    :param attribute_name: the subscription attribute name, must be in VALID_SUBSCRIPTION_ATTR_NAME
    :param attribute_value: the subscription attribute value
    :param topic_arn: the topic_arn of the subscription, needed to know if it is FIFO
    :raises InvalidParameterException
    :return:
    """
    if attribute_name not in sns_constants.VALID_SUBSCRIPTION_ATTR_NAME:
        raise InvalidParameterException("Invalid parameter: AttributeName")
    if attribute_name == "FilterPolicy":
        try:
            json.loads(attribute_value or "{}")
        except json.JSONDecodeError:
            raise InvalidParameterException(
                "Invalid parameter: FilterPolicy: failed to parse JSON."
            )
    elif attribute_name == "FilterPolicyScope":
        if attribute_value not in ("MessageAttributes", "MessageBody"):