### 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_
aws_client.sns.set_subscription_attributes(
                SubscriptionArn=subscription_arn,
                AttributeName="FilterPolicy",
                AttributeValue=json.dumps(nested_filter_policy),
            )
        snapshot.match("sub-filter-policy-nested-error-too-many-combinations", e.value.response)
        flat_filter_policy = {
            "key_a": ["value_one"],
            "key_b": ["value_two"],
            "key_c": ["value_three"],
            "key_d": ["value_four"],
            "key_e": ["value_five"],
            "key_f": ["value_six"],
        }
        # A 
        with pytest.raises(ClientError) as e:
            aws_client.sns.set_subscription_attributes(
                SubscriptionArn=subscription_arn,
                AttributeName="FilterPolicy",
                AttributeValue=json.dumps(flat_filter_policy),
            )
        snapshot.match("sub-filter-policy-max-attr-keys", e.value.response)
        flat_filter_policy = {"key_a": "value_one"}
        # R
        with pytest.raises(ClientError) as e:
            aws_client.sns.set_subscription_attributes(
                SubscriptionArn=subscription_arn,
                AttributeName="FilterPolicy",
                AttributeValue=json.dumps(flat_filter_policy),
            )