Supporting Message output for Java Function

[jerrypeng]

We have discussed and implemented Message as an input for a Java Function:
#4313

Perhaps its time to discuss supporting outputting Message as an option to that users can set any of the fields that is support in the Message interface for an outgoing message

We already have a message builder interface exposed via context, perhaps we can reuse it with some modification to just build a message for outputting instead of having to send it?

Maybe the API can be something like below:

public class TestFunction implements Function<String, Message<String>> {

    @Override
    public Message<String> process(String input, Context context) throws Exception {

        FunctionMessageBuilder<String> builder = context.newOutputMessage();

        Message message = builder.key("key").value("val").build();

        context.publish("topic", Schema.STRING, message); // either send the message

        return message; // or return from the function
    }
}

@srkukarni @sijie @merlimat @wolfstudy thoughts?

[srkukarni]

I like the build interface