feat(iot): backfill enum values in iot module (#33969)

### Description of changes

feat(iot): backfill enum values in iot module

CFN Doc: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iot-scheduledaudit.html#cfn-iot-scheduledaudit-dayofweek

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: xazhao

packages/@aws-cdk/aws-iot-alpha/lib/scheduled-audit.ts

@@ -165,6 +165,11 @@ export enum DayOfWeek {
    * Saturday
    */
   SATURDAY = 'SAT',
+
+  /**
+   * UNSET_VALUE
+   */
+  UNSET_VALUE = 'UNSET_VALUE',
 }
 
 /**