Add a flag to override (or not) the producer timestamp

[EtienneGuerlais]

Cloning a topic's content currently overrides the producer timestamp which is contained in the event's metadata with the cloning time.
While this can sometimes be the desired behavior, it is not always the case.
A flag could be added with three different behaviors:

• Set the timestamp to the cloning time
• Do not touch the timestamp
• Set the timestamp to the cloning time only for events that don't have one