Add local feature flag evaluation support

[marandaneto]

Summary

Add local feature flag evaluation support for server-side parity.

Why

sdk-specs marks local-feature-flag-evaluator and flag-definition-loader as backend-relevant. Server SDKs commonly support low-latency flag reads by polling local-evaluation definitions with a personal API key and falling back to remote evaluation when needed.

Current status

Elixir currently calls remote /flags (PostHog.FeatureFlags.flags/2, evaluate_flags/2) and does not appear to include local evaluation, flag definition polling, ETag/stale cache handling, or external definition cache providers.

Suggested scope

• Add config for local evaluation, including personal API key and poll interval.
• Fetch local-evaluation definitions from the dedicated endpoint.
• Cache definitions in memory, handle ETags/304s, and keep stale definitions on transient refresh failures.
• Evaluate person and group flags locally, including variants and payloads.
• Support remote fallback for inconclusive/unsupported cases.
• Consider external/shared cache provider support for distributed deployments.
• Add tests and docs.

References

• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/local-feature-flag-evaluator/spec.md
• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/flag-definition-loader/spec.md