### 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_
if self.message_attributes is None:
            self.message_attributes = {}
        if self.is_fifo:
            self.sequencer_number = str(get_next_sequence_number())
    def message_content(self, protocol: SnsMessageProtocols) -> str:
        """
        Helper function to retrieve the message content for the right protocol if the StructureMessage is `json`
        See https://docs.aws.amazon.com/sns/latest/dg/sns-send-custom-platform-specific-payloads-mobile-devices.html
        https://docs.aws.amazon.com/sns/latest/dg/example_sns_Publish_section.html
        :param protocol:
        :return: message content as string
        """
        if self.message_structure == "json":
            return self.message.get(protocol, self.message.get("default"))
        return self.message
    @classmethod
    def from_batch_entry(cls, entry: PublishBatchRequestEntry, is_fifo=False) -> "SnsMessage":
        return cls(
            type="Notification",
            message=entry["Message"],
            subject=entry.get("Subject"),
            message_structure=entry.get("MessageStructure"),
            message_attributes=entry.get("MessageAttributes"),
            message_deduplication_id=entry.get("MessageDeduplicationId"),
            message_group_id=entry.get("MessageGroupId"),
            is_fifo=is_fifo,
        )
class SnsSubscription(TypedDict):