Add Shutdown

[toddbaert]

Gracefully clean up providers during application shutdown.

Specification References:

• 1.6.1: The API MUST define a shutdown function.
• 1.6.2: The shutdown function MUST call the respective shutdown function on the active provider.
• 2.5.1: The provider MAY define a shutdown function.
• 2.5.2: After shutdown, the provider SHOULD revert to its uninitialized state.

Implementation Notes:

• Add shutdown() and shutdownAndWait() methods to OpenFeatureAPI
• Add optional shutdown() method to FeatureProvider protocol
• Shutdown must be called when providers are replaced (1.1.2.3)
• Important for flushing tracking data and closing connections