identitystore: Create identity center users

### Describe the feature

Currently, there's no way to create or manage Identity Center users through CDK. This is a significant gap in functionality: while CDK supports Identity Center groups, permission sets, and assignments, the inability to manage users means orgs must maintain a separate workflow (manual or via other tools) just for user management.

### Use Case

Most organizations using AWS Organizations with Identity Center need to manage:
User creation/deletion
User attributes (name, email, etc.)
User group memberships

### Proposed Solution

Add L1 constructs for Identity Store user management:
new identitystore.CfnUser(...);

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.154.1

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

Mac Sonoma 14.0

The issue does not name files or tests. Start by locating the existing Identity Center and identitystore L1 construct entry points, then compare how similar resources are represented and tested. Done means CDK exposes an identitystore.CfnUser construct covering Identity Center user management with the repository’s expected validation and tests.

Written by the indexing model from the issue text.