Support more flexible serialization in Log4jAppender

It would be nice to allow subclasses of the standard KafkfaLog4jAppender to be able to serialize the LoggingEvent however they chose, rather than always having to write out a string.

A possible use case - the one I'm interested in - allows implementors to convert the event to any sort of bytes. This means downstream consumers don't lose data based on the logging format, but instead can get the entire event to do with as they please