(rds): Make Aurora cluster instance class publicly available

### Describe the feature

- The `AuroraClusterInstance` class is [private](https://github.com/aws/aws-cdk/blob/eec900e90f38f34f896b22cf36cb225fc9c13cc8/packages/aws-cdk-lib/aws-rds/lib/aurora-cluster-instance.ts#L486). Thus, I can't find it in the node tree.
- The static [factory methods returns](https://github.com/aws/aws-cdk/blob/eec900e90f38f34f896b22cf36cb225fc9c13cc8/packages/aws-cdk-lib/aws-rds/lib/aurora-cluster-instance.ts#L400) `IClusterInstance`, which is not helpful and only has the `bind` method.
- I can't search for the `CfnDBInstance` instance because every construct ID value is a `Resource`, and every other prop is a token.

### Use Case

Sometimes, I need to get at instance-specific values, such as instance ID, or instance endpoint, e.g. for monitoring purposes.

### Proposed Solution

1. Make the `AuroraClusterInstance` class public and make it discoverable on the cluster.
2. Create the ability to pass the concrete `AuroraClusterInstance` instance that implements the `IClusterInstance` as the cluster reader and writer.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

latest

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

macOS

Start with packages/aws-cdk-lib/aws-rds/lib/aurora-cluster-instance.ts, especially AuroraClusterInstance and the static factory methods linked in the issue. Trace how the cluster exposes reader and writer instances, then check the public API exports and related construct tree behavior. Done means the concrete instance is publicly discoverable and usable for instance-specific values and reader/writer configuration.

Written by the indexing model from the issue text.