Include the reader host to the generated Aurora RDS cluster secret

### Name of the resource

AWS::SecretsManager::SecretTargetAttachment

### Resource name

_No response_

### Description

Include the reader host to the generated Secrets Manager secret for an RDS Aurora cluster.

Currently, the JSON it looks like this:

```json
{
"engine": "",
"dbClusterIdentifier": "",
"host": "",
"port": ""
"username": "",
"password": "",
"dbname": ""
}
```

### Use Case
It would be awesome to have the reader host in the secret, as we won't need to do string manipulation on our side:

```js
// What else can we do!? 😅
const readerHost = secret.host.replace('.cluster-', '.cluster-ro-');
```

### Proposed Solution
Just add a key conatining the readonly endpoint URL to the JSON (key name to your taste!), like so:

```json
{
"engine": "",
"dbClusterIdentifier": "",
"host": "",
"readerHost": "",
"port": ""
"username": "",
"password": "",
"dbname": ""
}
```

### Other Details

Related AWS CDK issues:
https://github.com/aws/aws-cdk/issues/11517
https://github.com/aws/aws-cdk/issues/22050

Start with AWS::SecretsManager::SecretTargetAttachment and inspect how the generated secret JSON is currently produced for an Aurora RDS cluster. Review the related AWS CDK issues for context and confirm the expected reader endpoint field; done means the secret includes the reader host without requiring string manipulation.

Written by the indexing model from the issue text.