PulsarKafkaProducer/Consumer do not set required configuration properties.

**Describe the bug**
```
new PulsarKafkaProducer<>(
props, new StringSerializer(), new StringSerializer<>());
```
fails with
```
org.apache.kafka.common.config.ConfigException: Missing required configuration "key.serializer" which has no default value.
```
Similarly, PulsarKafkaConsumer fails with missing `key.deserializer/value.serializer`.

**To Reproduce**
```
import java.util.Properties;
import org.apache.kafka.clients.producer.PulsarKafkaProducer;
import org.apache.kafka.common.serialization.StringSerializer;

public class ReproducePulsarBug {

public static void main(String[] args) {
Properties props = new Properties();
props.put("bootstrap.servers", "pulsar://localhost:49377");
// Only works if below two lines are commented in:
// props.put("key.serializer", StringSerializer.class.getName());
// props.put("value.serializer", StringSerializer.class.getName());
new PulsarKafkaProducer<>(
props, new StringSerializer(), new StringSerializer());
}
}
```

**Additional context**
```

org.apache.pulsar
pulsar-client-kafka-original
2.8.0

```

Start with the PulsarKafkaProducer and PulsarKafkaConsumer constructors used in the reproduction and trace how the supplied serializer and deserializer instances are handled. Run the provided Java example against the 2.8.0 adapter, then add regression coverage for construction without serializer or deserializer properties; done means both constructors no longer report those required configuration errors.

Written by the indexing model from the issue text.