feat(rds): limitless database cluster (#32151)

### Issue # (if applicable)

None

### Reason for this change

Amazon Aurora has released Postgres Limitless Database and AWS CDK does not support it. 
<https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/limitless.html>

### Description of changes

- Add `clusterScailabilityType` prop to `clusterProps`
- Instance creation will only be executed during the creation of a standard Aurora cluster
- Add some validations
  - Requirements for limitless database is documented at [here.](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/limitless-reqs-limits.html)


### Description of how you validated changes

Add both unit and integ tests

### 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: badmintoncryer