The firing Flezha.SDK.Tasklet.on_timer/3 receives: the timer's kind and
the scheduled_at timestamp its registration answered, the pair
Flezha.SDK.Ctx.cancel_timer/3 takes.
Summary
Types
@type kind() :: :processing_time | :event_time
@type t() :: %Flezha.SDK.TimerFiring{kind: kind(), scheduled_at: non_neg_integer()}