[ML] ScheduledEvent takes and stores ZonedDateTime but then discards the timezone #38620
Description
(Noticed while fixing #38506)
The ScheduledEvent class's constructor takes ZonedDateTime for both startTime and endTime. However, when serialising both on the wire and as XContent it ditches the time zone and converts to an Instant. It would make more sense to store Instant, take Instants in the constructor and return Instants from the accessors. Then it would be completely clear to all users that the class was not preserving the time zone.