rds: support Aurora Postgres Express

### Describe the feature

Support the newly launched Aurora PostgreSQL express configuration (--with-express-configuration) in CDK. This creates a VPC-less Serverless v2 cluster with an internet access gateway and IAM auth by default, all in a single API call.

### Use Case

Express configuration is ideal for dev/test and rapid prototyping where you want an Aurora PostgreSQL database without setting up VPC, subnets, or security groups. IaC users currently cannot use this from CDK or CloudFormation.

### Proposed Solution

This is blocked on CloudFormation first adding WithExpressConfiguration to AWS::RDS::DBCluster. Once that is available, CDK can expose it as a flag or a dedicated construct that omits VPC props and handles the internet access gateway endpoint.

### Other Information

- Blog post: https://aws.amazon.com/blogs/aws/announcing-amazon-aurora-postgresql-serverless-database-creation-in-seconds/
- Docs: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/CHAP_GettingStartedAurora.AuroraPostgreSQL.ExpressConfig.html
- The RDS API already supports this parameter, but the CloudFormation resource schema does not include it yet.

### Acknowledgements

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

### AWS CDK Library version (aws-cdk-lib)

2.248.0

### AWS CDK CLI version

2.1117.0

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

n/a

First verify whether CloudFormation's AWS::RDS::DBCluster schema now includes WithExpressConfiguration, since the issue identifies that as a blocker. Then inspect the CDK RDS cluster API and its tests to determine how the express option should be exposed; done means CDK can define the VPC-less Aurora PostgreSQL Express configuration with the required gateway endpoint and defaults.

Written by the indexing model from the issue text.