[Serve] Introduce scale_to_0_delay_seconds for autoscaler

[gaocegege]

We’ve introduced downscale_delay_seconds, which lets you adjust the scaling delay. In many cases, we want to scale efficiently from 1 to many or many to 1, but we might want a bit more of a delay when scaling all the way down to 0. Many Kubernetes auto scalers have a separate setting for scaling to 0.

For example, you could set downscale_delay_seconds to 1200s and scale_to_0_delay_seconds to 3600s to fine-tune the scaling's behavior.

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN