Add python-asyncio-fastapi example

[madhuchavva]

Summary

FastAPI example for the Python SDK's async integration pattern (growthbook >= 2.4.0, growthbook/growthbook-python#128): one lifespan-managed GrowthBookClient, an async Redis-backed sticky bucket service with a batched MGET get_all_assignments, and per-request UserContext. Falls back to an in-process store so it runs without Redis; docker-compose.yml provides Redis for the real path.

There was no Python example in this repo previously.

Test plan

• Smoke-tested with FastAPI TestClient against the SDK branch: lifespan init, /checkout returns the experiment variant, sticky assignment persisted, /healthz responsive, clean shutdown drain
• Manual run against a live GrowthBook instance once growthbook 2.4.0 is released (requirements.txt pins growthbook>=2.4.0)