(IAM): Policy L2 Construct helper methods

### Describe the feature

IPolicy has policyName only, looking for policyArn field as well.
IManagedPolicy has managedPolicyArn only, looking for managedPolicyName.

### Use Case

I want to take either IPolicy or IManagedPolicy and be able to extract both policy name and arn from it without custom implementation.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.132.0

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

Amazon Linux2

The issue names IPolicy and IManagedPolicy but no files or tests. Start by locating those interface definitions and their existing helper methods; done means either interface can provide both its policy name and ARN without a custom implementation.

Written by the indexing model from the issue text.