feat: Add async feature store interface and in-memory implementation

[jsonbailey]

Stacked on #451 (async support classes). Review/merge that first; this PR's diff is against that branch.

What

The async data-store foundation the async SDK builds on.

• interfaces.py — async interfaces: AsyncFeatureStore, AsyncReadOnlyStore (async get/all; the async analog of ReadOnlyStore), AsyncDataSourceUpdateSink, AsyncBigSegmentStore, AsyncInitializer, AsyncSynchronizer. Pure additions alongside the existing sync interfaces.
• async_feature_store.py — AsyncInMemoryFeatureStore. Holds no lock: every method reads/mutates its dict without awaiting in between, so it runs to completion without the event loop switching coroutines (the sync twin needs a ReadWriteLock because threads can preempt each other).
• testing/ — async test utilities: mock_async_components.py, async_feature_store_test_base.py, and test_async_in_memory_feature_store.py.

Why

Foundation for the async Python SDK (epic SDK-60). Subsequent slices (async evaluator, big segments, data sources, data systems, client) depend on these interfaces and the in-memory store.

Validation

• uv run pytest ldclient/testing/test_async_in_memory_feature_store.py — 17 passed
• uv run mypy ldclient — clean
• isort / pycodestyle — clean

Epic SDK-60.

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Note

Low Risk
Additive, experimental APIs and an in-memory reference implementation with broad test coverage; no changes to existing sync evaluation or storage paths.

Overview
Adds the experimental async data-store layer for the upcoming async Python SDK: new protocols/ABCs in interfaces.py and a default in-memory store, plus shared tests and mocks.

interfaces.py gains async counterparts to the sync data path—AsyncReadOnlyStore, AsyncFeatureStore (versioned get/all/init/upsert/delete, initialized, optional close), AsyncDataSourceUpdateSink, AsyncBigSegmentStore, and FDv2-style AsyncInitializer / AsyncSynchronizer (async fetch and async-generator sync). Existing sync types are unchanged.

async_feature_store.py introduces AsyncInMemoryFeatureStore, mirroring sync in-memory versioning and tombstone semantics but without a ReadWriteLock; methods do not await between dict reads/writes, so work completes on one event-loop turn.

testing/ adds AsyncFeatureStoreTestBase (parity with the sync store test suite), MockAsyncFeatureStore / MockAsyncEventProcessor / MockAsyncUpdateProcessor, and test_async_in_memory_feature_store.py wiring the in-memory implementation through that base.

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