### 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_
@pytest.mark.aws_validated
    @pytest.mark.skip_snapshot_verify(
        paths=[
            "$.invalid-json-redrive-policy.Error.Message",  # m
            "$.invalid-json-filter-policy.Error.Message",  # m
        ]
    )
    def test_validate_set_sub_attributes(
        self,
        sns_create_topic,
        sqs_create_queue,
        sqs_queue_arn,
        sns_create_s,
        snapshot,
        aws_client,
    ):
        topic_name = f"topic-{short_uid()}"
        queue_name = f"queue-{short_uid()}"
        topic_arn = sns_create_topic(Name=topic_name)["TopicArn"]
        queue_url = sqs_create_queue(QueueName=queue_name)
        subscription = sns_create_s(topic_arn=topic_arn, queue_url=queue_url)
        sub_arn = subscription["SubscriptionArn"]
        with pytest.raises(ClientError) as e:
            aws_client.sns.set_subscription_attributes(
                SubscriptionArn=sub_arn,
                AttributeName="FakeAttribute",
                AttributeValue="test-value",
            )
        snapshot.match("fake-attribute", e.value.response)
        with pytest.raises(ClientError) as e: