wafv2: Ability to add rules to an existing WebACL

### Describe the feature

Add additional rules to the existing WebAcl in our accounts. I am able to do this via the console, but unable to find a way to do it via CDK. This feature is something I would to request to be added.

### Use Case

In our enterprise, we have a cloud admin team provided BASE WebAcl.. they have mandated certain rules via this WebACL. Cloud teams are supposed to use this Web ACL and add additional rules to the existing WebAcl in their account. I am able to do this via the console, but unable to find a way to do it via CDK. This feature is something I would to request to be added.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.74.0

### Environment details (OS name and version, etc.)

AWS CDK V2, CloudFront, APIGW, ALB, Java

Start by locating the AWS CDK WAFv2 WebACL construct and its rule configuration entry points. Confirm how an existing WebACL is represented and determine the intended behavior for adding rules without replacing mandated rules; done means the feature supports this use case and has coverage for the resulting configuration.

Written by the indexing model from the issue text.