Separate logstash event creation from formatting operation

[talvey]

I've performed a very simple refactoring to expose the logstash json event to subclasses of JSONEventLayoutV1. The intent is to allow subclasses to override the format method and still be able to use the default logstash event creation logic.

So, something like the following:

    @Override
    public String format(LoggingEvent loggingEvent) {
       JSONObject logstashJson = createLogstashEvent(loggingEvent);
        String[] tags = new String[] {"CustomHardcodedTag"};
        logstashJson.put("tags", tags);

        return logstashJson.toString() + "\n";    
    }