OpenFeature provider support

Tracking issue for official OpenFeature support in the Python SDK — split out from the umbrella request PostHog/posthog#9609.

Status

✅ Official provider merged: feat(flags): add official PostHog OpenFeature provider #695 — ships as openfeature-provider-posthog (namespace openfeature.contrib.provider.posthog), resolving all five OpenFeature flag types
✅ Published to PyPI (currently Beta)
✅ Docs: Add OpenFeature (Python) feature flags provider docs posthog.com#18006