[Feature] Implement Multiton Pattern for Enhanced Instance Management in AsgardeoAuthClient

### Current Limitation

The `AsgardeoAuthClient ` currently relies on a public constructor, allowing for the uncontrolled creation of multiple instances without a centralized management system. Unlike the browser-specific implementations in the SDK ecosystem, there is no static mechanism to track, retrieve, or properly dispose of active client instances.

This lack of lifecycle management creates several issues:

- Inconsistency: It diverges from the architectural patterns used elsewhere in the SDK, leading to a fragmented developer experience.
- Resource Management: There is no standard way to destroy an instance or clear its session data, which poses risks for memory leaks.
- Multi-tenancy issues: Applications needing multiple isolated authentication contexts (e.g., distinct organization sessions) have no reliable way to manage and access specific instances by ID.
- Testing challenges: Without a way to destroyAllInstances(), ensuring a clean state between unit tests is difficult.

### Suggested Improvement

Refactor the AsgardeoAuthClient to implement the Multiton Pattern. This involves restricting direct instantiation and managing instances via a static internal map.

Specific changes recommended:

- Protected Constructor: Change the constructor visibility to protected to prevent direct usage of new AsgardeoAuthClient().
- Static Instance Map: Introduce a static Map to store active instances.
- Factory Method: Implement a static getInstance(id?: number) method that creates a new instance only if one does not exist for the given ID.
- Lifecycle Methods: Add utility methods to support proper cleanup:
- destroyInstance(id)
- destroyAllInstances()
- hasInstance(id)

### Please select the package issue is related to

@asgardeo/javascript

### Version

N/A

### Reporter Checklist

- [x] I have searched the existing issues and this is not a duplicate.
- [x] I have provided all the necessary information.
- [x] I have verified the improvement is not available in the latest version of the package.

Inizia individuando AsgardeoAuthClient nel package @asgardeo/javascript e esaminando le implementazioni dell’SDK specifiche per il browser a cui fa riferimento l’issue. Conferma il design necessario per la costruzione protetta, il recupero delle istanze tramite ID e la pulizia del ciclo di vita; il lavoro è completato quando getInstance, destroyInstance, destroyAllInstances e hasInstance supportano client isolati e uno stato dei test pulito.

Scritto dal modello di indicizzazione a partire dal testo della issue.