### 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_
"""
    In SNS, Subscription can be represented with only TopicArn, Endpoint, Protocol, SubscriptionArn and Owner, for
    example in ListSubscriptions. However, when getting a subscription with GetSubscriptionAttributes, it will return
    the Subscription object merged with its own attributes.
    This represents this merged object, for internal use and in GetSubscriptionAttributes
    https://docs.aws.amazon.com/cli/latest/reference/sns/get-subscription-attributes.html
    """
    TopicArn: topicARN
    Endpoint: str
    Protocol: SnsProtocols
    SubscriptionArn: subscriptionARN
    PendingConfirmation: Literal["true", "false"]
    Owner: Optional[str]
    SubscriptionPrincipal: Optional[str]
    FilterPolicy: Optional[str]
    FilterPolicyScope: Literal["MessageAttributes", "MessageBody"]
    RawMessageDelivery: Literal["true", "false"]
    ConfirmationWasAuthenticated: Literal["true", "false"]
    SubscriptionRoleArn: Optional[str]
class SnsStore(BaseStore):