feat(codestarnotifications): add createdBy property for notification rule (#30780)

### Issue # (if applicable)

### Reason for this change
The `createdBy` property existed in the L1 construct but was not present in the L2 construct

### Description of changes
- Add the `createdBy` property for `NotificationRule`, which was missing in the L2 construct.

### Description of how you validated changes
I Added a unit test for codestarnotifications and integration tests for pipeline and codestarnotifications

### Checklist
- [x] 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)

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

Author: yuppe-kyupeen