feat: implement scheduling mechanism for prebuilds (#18126)

Closes https://github.com/coder/internal/issues/312
Depends on https://github.com/coder/terraform-provider-coder/pull/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.

---------

Co-authored-by: Danny Kopping <danny@coder.com>

coderd/schedule/cron/cron.go

@@ -71,6 +71,29 @@ func Daily(raw string) (*Schedule, error) {
 	return parse(raw)
 }
 
+// TimeRange parses a Schedule from a cron specification interpreted as a continuous time range.
+//
+// For example, the expression "* 9-18 * * 1-5" represents a continuous time span
+// from 09:00:00 to 18:59:59, Monday through Friday.
+//
+// The specification consists of space-delimited fields in the following order:
+// - (Optional) Timezone, e.g., CRON_TZ=US/Central
+// - Minutes: must be "*" to represent the full range within each hour
+// - Hour of day: e.g., 9-18 (required)
+// - Day of month: e.g., * or 1-15 (required)
+// - Month: e.g., * or 1-6 (required)
+// - Day of week: e.g., * or 1-5 (required)
+//
+// Unlike standard cron, this function interprets the input as a continuous active period
+// rather than discrete scheduled times.
+func TimeRange(raw string) (*Schedule, error) {
+	if err := validateTimeRangeSpec(raw); err != nil {
+		return nil, xerrors.Errorf("validate time range schedule: %w", err)
+	}
+
+	return parse(raw)
+}
+
 func parse(raw string) (*Schedule, error) {
 	// If schedule does not specify a timezone, default to UTC. Otherwise,
 	// the library will default to time.Local which we want to avoid.
@@ -155,6 +178,24 @@ func (s Schedule) Next(t time.Time) time.Time {
 	return s.sched.Next(t)
 }
 
+// IsWithinRange interprets a cron spec as a continuous time range,
+// and returns whether the provided time value falls within that range.
+//
+// For example, the expression "* 9-18 * * 1-5" represents a continuous time range
+// from 09:00:00 to 18:59:59, Monday through Friday.
+func (s Schedule) IsWithinRange(t time.Time) bool {
+	// Truncate to the beginning of the current minute.
+	currentMinute := t.Truncate(time.Minute)
+
+	// Go back 1 second from the current minute to find what the next scheduled time would be.
+	justBefore := currentMinute.Add(-time.Second)
+	next := s.Next(justBefore)
+
+	// If the next scheduled time is exactly at the current minute,
+	// then we are within the range.
+	return next.Equal(currentMinute)
+}
+
 var (
 	t0   = time.Date(1970, 1, 1, 1, 1, 1, 0, time.UTC)
 	tMax = t0.Add(168 * time.Hour)
@@ -263,3 +304,18 @@ func validateDailySpec(spec string) error {
 	}
 	return nil
 }
+
+// validateTimeRangeSpec ensures that the minutes field is set to *
+func validateTimeRangeSpec(spec string) error {
+	parts := strings.Fields(spec)
+	if len(parts) < 5 {
+		return xerrors.Errorf("expected schedule to consist of 5 fields with an optional CRON_TZ=<timezone> prefix")
+	}
+	if len(parts) == 6 {
+		parts = parts[1:]
+	}
+	if parts[0] != "*" {
+		return xerrors.Errorf("expected minutes to be *")
+	}
+	return nil
+}