Add new serve autoscaling parameter `scaling_function`

[Stack-Attack]

Why are these changes needed?

Currently, the serve autoscaler makes scaling decisions only based on the most recent Serve Controller computation, even if the serve controller has made many scaling calculations over the scaling delay period. This results in poor autoscaling when clusters utilize long upscale/downscale delays.

Related issue number

https://github.com/ray-project/ray/issues/46497

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• Testing Strategy
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