feat: implement expiration policy logic for prebuilds (#17996)

## Summary 

This PR introduces support for expiration policies in prebuilds. The TTL
(time-to-live) is retrieved from the Terraform configuration
([terraform-provider-coder
PR](https://github.com/coder/terraform-provider-coder/pull/404)):
```
prebuilds = {
	  instances = 2
	  expiration_policy {
		  ttl = 86400
	  }
  }
```
**Note**: Since there is no need for precise TTL enforcement down to the
second, in this implementation expired prebuilds are handled in a single
reconciliation cycle: they are deleted, and new instances are created
only if needed to match the desired count.

## Changes

* The outcome of a reconciliation cycle is now expressed as a slice of
reconciliation actions, instead of a single aggregated action.
* Adjusted reconciliation logic to delete expired prebuilds and
guarantee that the number of desired instances is correct.
* Updated relevant data structures and methods to support expiration
policies parameters.
* Added documentation to `Prebuilt workspaces` page
* Update `terraform-provider-coder` to version 2.5.0:
https://github.com/coder/terraform-provider-coder/releases/tag/v2.5.0

Depends on: https://github.com/coder/terraform-provider-coder/pull/404
Fixes: https://github.com/coder/coder/issues/17916

coderd/provisionerdserver/provisionerdserver.go

@@ -2059,23 +2059,26 @@ func InsertWorkspacePresetsAndParameters(ctx context.Context, logger slog.Logger
 
 func InsertWorkspacePresetAndParameters(ctx context.Context, db database.Store, templateVersionID uuid.UUID, protoPreset *sdkproto.Preset, t time.Time) error {
 	err := db.InTx(func(tx database.Store) error {
-		var desiredInstances sql.NullInt32
+		var desiredInstances, ttl sql.NullInt32
 		if protoPreset != nil && protoPreset.Prebuild != nil {
 			desiredInstances = sql.NullInt32{
 				Int32: protoPreset.Prebuild.Instances,
 				Valid: true,
 			}
+			if protoPreset.Prebuild.ExpirationPolicy != nil {
+				ttl = sql.NullInt32{
+					Int32: protoPreset.Prebuild.ExpirationPolicy.Ttl,
+					Valid: true,
+				}
+			}
 		}
 		dbPreset, err := tx.InsertPreset(ctx, database.InsertPresetParams{
-			ID:                uuid.New(),
-			TemplateVersionID: templateVersionID,
-			Name:              protoPreset.Name,
-			CreatedAt:         t,
-			DesiredInstances:  desiredInstances,
-			InvalidateAfterSecs: sql.NullInt32{
-				Int32: 0,
-				Valid: false,
-			}, // TODO: implement cache invalidation
+			ID:                  uuid.New(),
+			TemplateVersionID:   templateVersionID,
+			Name:                protoPreset.Name,
+			CreatedAt:           t,
+			DesiredInstances:    desiredInstances,
+			InvalidateAfterSecs: ttl,
 		})
 		if err != nil {
 			return xerrors.Errorf("insert preset: %w", err)