### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

I am running an SNS topic that publishes to a lambda function. The localstack code reads a type byte object from the lambda function invocation payload [here](https://github.com/localstack/localstack/blob/master/localstack/services/sns/publisher.py#L187) and then tries to dump the result into json [here](https://github.com/localstack/localstack/blob/master/localstack/services/sns/publisher.py#L863) which results in the following error:
```
sns_pub_0] l.services.sns.publisher   : Unable to run Lambda function on SNS message: Object of type bytes is not JSON serializable
```

### Expected Behavior

The payload bytes should be converted to a string before being dumped to json. The error should not be thrown.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

docker-compose up
(Using the standard docker-compose.yaml)

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

```
awslocal sns publish --message file://sns/message.json --topic-arn arn:aws:sns:us-east-1:000000000000:<topic_name>
```

### Environment

```markdown
- OS: macOS Ventura 13.2.1
- LocalStack: latest
```


### Anything else?

_No response_
"""
    The application publisher is responsible for publishing the SNS message directly to a registered SNS application
    endpoint, without it being subscribed to a topic.
    See `ApplicationTopicPublisher` for more information about Application Endpoint publishing.
    """
    def _publish(self, context: SnsPublishContext, endpoint: str):
        message = self.prepare_message(context.message, endpoint)
        cache = context.store.platform_endpoint_messages[endpoint] = (
            context.store.platform_endpoint_messages.get(endpoint) or []
        )
        cache.append(message)
        if (
            config.LEGACY_SNS_GCM_PUBLISHING
            and get_platform_type_from_endpoint_arn(endpoint) == "GCM"
        ):
            self._legacy_publish_to_gcm(context, endpoint)
        
        
        
        
    def prepare_message(self, message_context: SnsMessage, endpoint: str) -> Union[str, Dict]: