[improvement] Functions: ability to produce KeyValue<GenericRecord,GenericRecord> output

**Is your enhancement request related to a problem? Please describe.**
Currently (Pulsar 2.10) if I code a simply Function like this:

```
@Slf4j
public class MyFunction implements Function> {

@Data
@AllArgsConstructor
public static final class MyKey {
String name;
}
@Data
@AllArgsConstructor
public static final class MyValue {
int age;
}

@Override
public KeyValue apply(GenericObject genericObject) {
log.info("apply to {} {}", genericObject, genericObject.getNativeObject());
return new KeyValue<>(new MyKey("foo"), new MyValue(543));
}
}
```

I am able to successfully consume a KeyValue topic (like a topic written by Debezium or the Kafka Source), but I cannot produce to the output topic, because Pulsar sets the key and value type of the KeyValue to byte[].
It seems to ignore the Generic Type of the KeyVale Output datatype .

This is the error we can see

```
8:00:30.993 [public/default/ciao2-0] ERROR org.apache.pulsar.functions.instance.JavaInstanceRunnable - [public/default/ciao2:0] Uncaught exception in Java Instance
java.lang.ClassCastException: class MyFunction$MyKey cannot be cast to class [B (MyFunction$MyKey is in unnamed module of loader org.apache.pulsar.common.nar.NarClassLoader @70f822e; [B is in module java.base of loader 'bootstrap')
at org.apache.pulsar.client.impl.schema.BytesSchema.encode(BytesSchema.java:28) ~[pulsar-client-original.jar:2.8.0.1.1.29-SNAPSHOT]
at org.apache.pulsar.common.schema.KeyValue.encode(KeyValue.java:107) ~[java-instance.jar:?]
```

**Describe the solution you'd like**
No error happens, out-of-the-box. Data is encoded with KeyValue or KeyValue (and SEPARATE encoding possibly).

**Describe alternatives you've considered**
Setting a schema on the output topic before creating the function (I didn't test this, we should add integration tests for this case as well)