feat: add scheduling configuration for prebuilds

[evgeniy-scherbina]

Relates to coder/internal#312
Needed for coder/coder#18126

Exposes an autoscaling field under coder_workspace_preset:

data "coder_workspace_preset" "us-nix" {
  ...
  
  prebuilds = {
	  instances = 0                  # default to 0 instances
	  
	  scheduling {
		  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
		  }
	  }
  }
}

[spikecurtis]

I really don't think we should call this "autoscaling" as that already has a specific meaning in Cloud circles which this is not.

Autoscaling is adjusting instance counts dynamically based on measured load.

This is adjusting instance counts based on a schedule. Some possible names:

• Scale sceduling
• Schedule-scaling
• Pool scheduling
• Prebuild scheduling

[dannykopping]

I really don't think we should call this "autoscaling" as that already has a specific meaning in Cloud circles which this is not.

Autoscaling is adjusting instance counts dynamically based on measured load.

This is adjusting instance counts based on a schedule. Some possible names:

• Scale sceduling
• Schedule-scaling
• Pool scheduling
• Prebuild scheduling

I agree with @spikecurtis; autoscaling does indeed feel muddy in retrospect.
I'll offer scheduling as a another alternative.

[dannykopping]

This error message could be more helpful. We know which aspect of the schedules overlap, so let's help template authors out by telling them.

[evgeniy-scherbina]

It means everything (minutes, hours and days) overlap.

[dannykopping]

This is excellent work @evgeniy-scherbina!

I really like how clean the code is, how complete the code-coverage, and how clear the intentions are.

[evgeniy-scherbina]

I really don't think we should call this "autoscaling" as that already has a specific meaning in Cloud circles which this is not.

Autoscaling is adjusting instance counts dynamically based on measured load.

This is adjusting instance counts based on a schedule. Some possible names:

• Scale sceduling
• Schedule-scaling
• Pool scheduling
• Prebuild scheduling

@spikecurtis I renamed everything to scheduling