aws_rds: add addRotationMultiUser and addRotationSingleUser methods to database interface

### Describe the feature

The methods addRotationMultiUser and addRotationSingleUser are only available when you create the database cluster (in my case ServerlessCluster). However, these methods are not available when you import the database cluster (IServerlessCluster).

### Use Case

I share the database cluster with all my applications and it's in one stage. The applications using the shared database cluster are in different stages so I cannot use prop drilling. I need to import the database cluster and I cannot use these methods.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.29.1

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

MacOS 12.4

Start in the aws_rds database interfaces and the existing ServerlessCluster implementations for addRotationMultiUser and addRotationSingleUser. Compare those definitions with IServerlessCluster and its tests; done means imported database clusters expose both methods with behavior consistent with newly created clusters.

Written by the indexing model from the issue text.