[Feature Request] Add option to specify produced messages timestamp

[jankalinic]

Summary

Add an option to the client builder to allow specifying a custom timestamp for produced Kafka messages, rather than always defaulting to the current wall-clock time.

Motivation

When writing tests, it's often necessary to simulate messages that appear to have been produced at a specific point in the past (or future). Currently, there's no way to control the message timestamp through the clients.
Without this capability, users have to either mock system clocks, or use Kafka consumer binaries logic.