Kafka transactions are unsupported

In the latest node-rdkafka, I see that the C++ implementation of the producer does not implement or call transaction-related methods, and therefore the JavaScript side of things doesn't have any transaction-related methods. It would be nice if the library supported these, as librdkafka does under the hood.

Start with the C++ producer implementation and trace how its capabilities are exposed on the JavaScript side. Compare the missing transaction-related methods with the transaction support provided by librdkafka. Done means the library exposes and supports Kafka producer transactions end to end, with behavior verified against the underlying librdkafka capability.

Written by the indexing model from the issue text.