(aws-rds): headless clusters

### Describe the feature

> A headless secondary Aurora DB cluster is one without a DB instance.
>
> This type of configuration can lower expenses for an Aurora global database. In an Aurora DB cluster, compute and storage are decoupled. Without the DB instance, you're not charged for compute, only for storage. If it's set up correctly, a headless secondary's storage volume is kept in-sync with the primary Aurora DB cluster.

[Creating a headless Aurora DB cluster in a secondary Region](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-global-database-getting-started.html#aurora-global-database-attach.console.headless)

### Use Case

Currently, there don't seem to be any L2/L3 constructs available to conveniently deploy a headless RDS cluster. Although there are solutions involving implementing this by hand, these seem intricate and involve mixing with L1 constructs.

### Proposed Solution

It would be nice to have an L2/L3 construct to neatly and conveniently provision headless RDS clusters.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.24.1 (build 585f9ca)

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

N/A

Start by reading the linked AWS documentation on creating a headless Aurora cluster and examining the AWS CDK's current RDS L2/L3 construct coverage. Determine the API and resource behavior needed to provision a secondary Aurora cluster without a DB instance; done means a convenient construct supports this configuration without requiring users to mix in L1 constructs.

Written by the indexing model from the issue text.