feat: Create GlobalHookManager.

[NeaguGeorgiana23]

This PR

• Implements GlobalHookManager as an independent, thread-safe singleton to store and manage global hooks using std::shared_mutex.
• Decouples global hook management from OpenFeatureAPI, mirroring the design pattern established by GlobalContextManager. This allows ClientAPI to access global hooks during flag evaluation without introducing a circular dependency between :client_api and :openfeature_api in Bazel.
• Updates OpenFeatureAPI to delegate AddHook, AddHooks, GetHooks, and Shutdown (clearing hooks) to GlobalHookManager::GetInstance().
• Ensures nullptr hook pointers are filtered out and registration order is strictly preserved.
• Adds comprehensive unit tests in test/global_hook_manager_test.cpp covering initial state, single/batch hook registration, order preservation, null filtering, hook clearing, and multithreaded reader/writer stress testing.

Related Issues

Fixes #133

Follow-up Tasks

• Implement the hook execution lifecycle pipeline (before, after, error, finally) inside ClientAPI::EvaluateFlag.
• Add e2e tests for the complete hook evaluation pipeline.

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