Handle timestamp types in backups

[matyaskuti]

What is currently missing?

Prompted by a discussion on a PR replacing Kafka consumer implementations with confluent-kafka, it turns out we do not back up the timestamp types of messages/records. (The kafka-python library had this as a separate field ConsumerRecord.timestamp_type; confluent-kafka has a 2-tuple for Message timestamps.)

How could this be improved?

• Should timestamp types be backed up and restored?
• The implementation seems relatively straightforward, the current timestamps could be extended with the type

Is this a feature you would work on yourself?

• [ ] I plan to open a pull request for this feature