Support for transactional consume-transform-produce loop

[nick-zh]

For the use case where you have the following:

• consumer reading messages
• processing those messages
• producing a result of those processed messages
• all the above being part of a transaction

We need to be able for the consumer offsets to be part of the transaction, this can be achieved by using rd_kafka_send_offsets_to_transaction for which we should add support for.

Is related to #359

[donatascn]

It is odd that this feature is not being upvoted :)

[nizacegodk]

We would also benefit from having this feature in our projects.