feat: Add async hook, plugin, and flag tracker

[jsonbailey]

Overview

This slice adds the async counterparts of the SDK's hook and plugin extension points, plus the async flag tracker used by the async client:

• AsyncHook (ldclient/hook.py) — abstract base for async hooks. before_evaluation / after_evaluation are coroutines awaited by the async client.
• AsyncPlugin (ldclient/plugin.py) — abstract base for async plugins. register() is a coroutine (allowing async initialization such as connecting to telemetry backends); get_hooks() remains synchronous and returns AsyncHook instances.
• AsyncFlagTrackerImpl / AsyncFlagValueChangeListener (ldclient/impl/async_flag_tracker.py) — the async flag tracker implementation.

Unit tests are added in ldclient/testing/test_async_hooks.py and ldclient/testing/test_async_flag_tracker.py.

The async public classes carry the experimental .. caution:: block, matching the rest of the async surface.

This PR depends only on already-merged foundation work and can be reviewed independently.

Tracked internally: SDK-2737

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Note

Low Risk
Additive experimental async APIs and new implementation files; no changes to sync client evaluation or existing plugin contracts.

Overview
Adds experimental async extension points and flag-change tracking for AsyncLDClient, paralleling the existing sync Hook, Plugin, and FlagTrackerImpl.

AsyncHook in hook.py mirrors Hook but with async before_evaluation / after_evaluation coroutines the async client will await. AsyncPlugin in plugin.py adds an async register() for AsyncLDClient and synchronous get_hooks() returning AsyncHook instances only (with AsyncHook imported alongside Hook).

AsyncFlagTrackerImpl and AsyncFlagValueChangeListener implement async flag-value change listeners: baseline evaluation on subscribe (no initial callback), re-evaluation on matching FlagChange events, deduplication when the value is unchanged, and scheduling via AsyncCallbackScheduler so notifications work from worker threads without a running loop.

Unit tests cover hook dispatch order (before forward, after reverse), per-hook isolated hook data, and tracker add/remove/thread-notify behavior.

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