FEATURE REQUEST: ScheduleAt::Delay

[joshua-spacetime]

From @SteveBoytsun:

it would be nice to have a ScheduleAt::Delay option that works like ScheduleAt::Time but with relative offset (so we can schedule a one-off timer like so ScheduleAt::Delay(Duration::from_secs(secs)) instead of ScheduleAt::Time(Timestamp::now().checked_add(Duration::from_secs(secs)).unwrap().into_micros_since_epoch()))

[bfops]

We will implement this as a helper method (not a new enum variant).