Encode namespace_id in sent_notifications reply_key

Mario Celirequested to merge
594664-encode-namespace_id into master

What does this MR do and why?

We need to encode the sharding key for SentNotification in the reply_key token. THis will be used later in the imail ingestion service to identify a cell an email has been sent to. Still under discussion at #592201 (comment 3189167020) but might also be used for unsubscribe urls. The sooner we implement this change, the better as we have decided not to claim this value in the topology service in favor of using this approach, knowing that the only instances that will break are those that have been moved to a different cell prior to this change.

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Related to #594664

Edited by Mario Celi

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