Is your feature request related to a problem? Please describe.
During a scheduled in-game events or new version releases, we see pretty rapid spikes in usage of either an already high use fleet or a newer unused fleet. Both of these events we know the timing of, and our current options are either:

1. Prescale agressively: this works but means that unless we are building scheduling logic ourselves to undo the additional scale afterwards, we're paying for a lot of unused capacity.
2. Webhook autoscaler: this is a viable solution, but requires us to build a service to do so.

Describe the solution you'd like
Introduce the concept of scheduled overrides that contain the following:

1. a start time(in UTC)
2. an end time(in UTC)
3. a priority int(higher the better, much like PriorityClasses)
4. a buffer autoscaler block

Then on autoscaling evaluation:

1. collect those overrides for which we are between the start and end time
   a. if there are no matching overrides, just use the default autoscaling rule
2. of those select the highest priority
3. apply that buffer autoscaling rule instead of the default

This would allow us to set special scaling windows for events or new version releases. A further extension could be to allow recurring windows to do time of day scheduling so that we could have a buffer window in the off hours and a percentage during higher usage, which could help with issues like that described in #2504

Describe alternatives you've considered
As described at the top, we can either prescale agressively, which either results in us adjusting the autoscaler directly, or using the webhook autoscaler.