Enhance ConfigurationManager with event handling

[GeoK]

Add extensible event hooks to ConfigurationManager to empower developers to handle custom caching (including L2/distributed cache) and other advanced scenarios via configuration event handlers.

Proposal:
Expose event hooks (e.g., via IConfigurationEventHandler) for:
• Pre-retrieval (to allow cache checks or custom fetch logic)
• Post-update (to allow cache population or notifications)