### 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.fixture
    def sns_primary_client(self, aws_client):
        return aws_client.sns
    @pytest.fixture
    def sns_secondary_client(self, secondary_aws_client):
        return secondary_aws_client.sns
    @pytest.fixture
    def sqs_primary_client(self, aws_client):
        return aws_client.sqs
    @pytest.fixture
    def sqs_secondary_client(self, secondary_aws_client):
        return secondary_aws_client.sqs
    def test_cross_account_access(self, sns_primary_client, sns_secondary_client):
        # C
        # T
        #
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        topic_name = f"topic-{short_uid()}"
        topic_arn = sns_primary_client.create_topic(Name=topic_name)["TopicArn"]
        assert sns_secondary_client.set_topic_attributes(
            TopicArn=topic_arn, AttributeName="DisplayName", AttributeValue="xenon"
        )