TransactionalProducer to allow uninterruptible after commit effect

[domartynov]

With current API, it's not clear how to implement graceful shutdown of an application where e.g. a published state needs to be committed to DB after a successful commit to Kafka without interruption.

[erikvanoosten]

Committing offsets to a database and to Kafka is not recommended as it can cause consistency issues.

For example, when committing first to Kafka, and then to the database, it is possible that the database commit is lost. The next consumer however will not pick up these records again because they are already committed.
When committing first to the database and then to Kafka, it is possible that the Kafka commit is lost. This means that records can be processed twice. This might be fine but at-least-once is already possible without transactional commits.

In the forthcoming zio-kafka 3.0 release using transactional producer is much improved. It includes documentation that describes how to complete the transaction while the application is shutting down (see changes in #1432).