(aws-servicecatalogappregistry-alpha): Add retrieving of attributes from construct

### Describe the feature

Add an `attributes` property or a `getAttributes()` or `getAttribute(path: string)` method to allow retrieval of the attributes when importing the construct using the `fromAttributeGroupArn` method.

### Use Case

I have a bunch of attributes I want to share with multiple accounts but at the moment I have to use lots of SSM parameters for each account due to the way strings/lists/json is resolved I've tried various ways of doing it with Lazy and such but without luck. Having a way to share configuration objects from a centralized place would be great and this is the closest service i have seen to be able to do that cleanly.

### 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

### AWS CDK Library version (aws-cdk-lib)

2.215.0

### AWS CDK CLI version

2.1029.2

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

6.6.87.2-microsoft-standard-WSL2 Ubuntu 24.04.2 LTS

Start by locating the TypeScript construct and its fromAttributeGroupArn entry point, then inspect how imported attribute groups are represented and covered by existing tests. Done means exposing a documented way to retrieve imported attributes by object, key, or path, with tests demonstrating the supported access pattern.

Written by the indexing model from the issue text.