### 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_
self, context: RequestContext, subscription_arn: subscriptionARN
    ) -> GetSubscriptionAttributesResponse:
        store = self.get_store(account_id=context.account_id, region_name=context.region)
        sub = store.subscriptions.get(subscription_arn)
        if not sub:
            raise NotFoundException("Subscription does not exist")
        removed_attrs = ["sqs_queue_url"]
        if "FilterPolicyScope" in sub and "FilterPolicy" not in sub:
            removed_attrs.append("FilterPolicyScope")
        elif "FilterPolicy" in sub and "FilterPolicyScope" not in sub:
            sub["FilterPolicyScope"] = "MessageAttributes"
        attributes = {k: v for k, v in sub.items() if k not in removed_attrs}
        return GetSubscriptionAttributesResponse(Attributes=attributes)
    def publish(