fix: Shutdown calls providers' shutdown

[fabriziodemaria]

Addressing #124.

The fix might not be that simple, if we want to execute the shutting down of providers in a background threads while leaving all the APIs available at runtime. My concern here is the possible race condition or simply operational overlap of shut downs and, for example, the in-flight evaluation operations.

Any suggestion from the Rust community is appreciated here.

[gemini-code-assist]

Summary of Changes

Hello @fabriziodemaria, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the resource management within the OpenFeature API by introducing a formal shutdown mechanism for FeatureProvider implementations. It ensures that providers can gracefully release resources when they are no longer needed, either due to being replaced or during application shutdown. The changes involve extending the FeatureProvider trait, updating the ProviderRegistry to manage the shutdown lifecycle, and adding robust tests to validate this behavior, thereby enhancing the overall stability and reliability of the system.

Highlights

• FeatureProvider Shutdown Trait: A new shutdown asynchronous method has been added to the FeatureProvider trait, allowing providers to implement graceful termination logic and resource cleanup.
• Provider Lifecycle Management: The ProviderRegistry now ensures that a provider's shutdown method is invoked when it is replaced by a new provider (either default or named) or when the entire registry is cleared.
• Asynchronous and Idempotent Shutdown: A FeatureProviderWrapper and ProviderEntry have been introduced to manage provider instances, ensuring that the shutdown method is called asynchronously in the background and only once per provider, using AtomicBool for state management.
• Enhanced Test Coverage: New tokio::test cases have been added and existing ones modified to explicitly verify that provider shutdown methods are correctly invoked when a provider is swapped out and when the global API shutdown is triggered.

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[gemini-code-assist]

Code Review

This pull request introduces a shutdown mechanism for providers, which is a great addition for resource management. The implementation correctly identifies places where provider shutdown is needed.

I've found a couple of issues with the current implementation:

1. There's a compilation error in ProviderRegistry::set_default due to incorrect lock handling.
2. The api.shutdown() method doesn't wait for providers to complete their shutdown process. This "fire-and-forget" behavior can be surprising and lead to resource leaks if the application exits before shutdown is complete.

I've provided detailed comments with suggestions to fix these issues. The proposed changes will make the shutdown behavior more robust and predictable.