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 APIs and implementation behind the async client surface; no changes to sync evaluation paths, with behavior covered by new unit tests.

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

AsyncHook and AsyncPlugin introduce coroutine-based before_evaluation / after_evaluation and register(), with sync get_hooks() returning AsyncHook instances. New public types are marked with the experimental caution block.

AsyncFlagTracker (interface) and AsyncFlagTrackerImpl implement async per-flag value listeners: subscription awaits an initial evaluation for a baseline, re-evaluates on config changes, and notifies only when the value differs. Work is scheduled via AsyncCallbackScheduler so notifications from worker threads remain safe.

Docs clarify that AsyncReadOnlyStore is experimental and that BigSegmentStoreStatusProvider.status is non-blocking on the async client (available=False until the first background poll). Unit tests cover hook dispatch semantics and flag-tracker behavior, including cross-thread notify.

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