Closes coder/internal#312
Depends on coder/terraform-provider-coder#408

This PR adds support for defining an autoscaling block for prebuilds, allowing number of desired instances to scale dynamically based on a schedule.

Example usage:

data "coder_workspace_preset" "us-nix" {
  ...
  
  prebuilds = {
    instances = 0                  # default to 0 instances
    
    scheduling = {
      timezone = "UTC"             # a single timezone is 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
      }
    }
  }
}

Behavior

• Multiple schedule blocks per prebuilds block are supported.
• If the current time matches any defined autoscaling schedule, the corresponding number of instances is used.
• If no schedule matches, the default instance count (prebuilds.instances) is used as a fallback.

Why

This feature allows prebuild instance capacity to adapt to predictable usage patterns, such as:

• Scaling up during business hours or high-demand periods
• Reducing capacity during off-hours to save resources

Cron specification

The cron specification is interpreted as a continuous time range.

For example, the expression:

* 9-18 * * 1-5

is intended to represent a continuous range from 09:00 to 18:59, Monday through Friday.

However, due to minor implementation imprecision, it is currently interpreted as a range from 08:59:00 to 18:58:59, Monday through Friday.

This slight discrepancy arises because the evaluation is based on whether a specific point in time falls within the range, using the github.com/coder/coder/v2/coderd/schedule/cron library, which performs per-minute matching rather than strict range evaluation.