Serialization beyond JSON

[akucherenko]

Current implementation of RdKafka transport supports a single way to serialize/unserialize payloads, that is either json_encode of PsrMessage RdKafkaProducer.php or json_decode and further PsrMessage population RdKafkaConsumer.php. In this case, consumer and producer are only mutually compatible and consuming messages of different format is hardly possible. E.g. no chance to plug into an Avro-encoded stream.

An option here might be making serializer configurable. Or provide abstraction around transforming payload into an object. Maybe there are unrecognized reasons behind current implementation. Anyway, I'd like to discuss that and provide help if needed.