feat: add autoscaling configuration for prebuilds

[evgeniy-scherbina]

Relates to coder/internal#312

Exposes an autoscaling field under coder_workspace_preset:

data "coder_workspace_preset" "us-nix" {
  ...
  
  prebuilds = {
	  instances = 0                  # default to 0 instances
	  
	  autoscaling {
		  timezone = "UTC"             # only a single timezone may be used for simplicity
		  
		  # scale to 3 instances during the work week
		  schedule {
		    cron = "* 8-18 * * 1-5"    # from 8AM-6:59PM, Mon-Fri, UTC
		    instances = 3              # scale to 3 instances
		  }
		  
		  # scale to 1 instance on Saturdays for urgent support queries
		  schedule {
		    cron = "* 8-14 * * 6"      # from 8AM-2:59PM, Sat, UTC
		    instances = 1              # scale to 1 instance
		  }
	  }
  }
}