RemoteEval for Sync and Async Clients

[madhuchavva]

Add remote evaluation support

Summary

Adds remoteEval / remote_eval to both the sync GrowthBook class and the async GrowthBookClient, matching the JS SDK's behavior. The SDK POSTs the user's targeting context to {api_host}/api/eval/{client_key} on a self-hosted GrowthBook proxy/edge and runs its existing local eval_feature over the user-filtered response.

What's included

• Options — 3 new fields with defaults: remote_eval, cache_key_attributes, remote_eval_cache_size. Zero impact on existing callers.
• FeatureRepository — POST /api/eval/{client_key} plumbing; payload-aware cache key (matches JS: {clientKey}||{json({ca, fv, url})}; forcedFeatures deliberately excluded).
• Sync GrowthBook — 5 validation guards (missing client_key, decryption_key, sticky_bucket_service, stale_while_revalidate, *.growthbook.io host); new set_url / set_forced_variations setters that trigger a blocking refetch when remote eval is on; SSE branching; skip global callback registration in remote-eval mode to prevent singleton cross-pollution between instances.
• Async GrowthBookClient — per-UserContext POST with an LRU-bounded cache (default 1000), inflight coalescing so concurrent identical evals issue one POST, preload_remote_eval() to warm the cache at request boundaries, SSE → cache-flush invalidation.
• rule.tracks — fires trackingCallback for each deferred experiment-tracking event the proxy attaches to force rules (matches JS in packages/sdk-js/src/core.ts).
• Closes Feature/remote eval #63 (replaces stale PR).

Test plan

• 34 new tests in tests/test_remote_eval.py — POST URL/body shape, all 5 validation guards, setter-triggered refetches, cache hit/miss on attribute change, cache_key_attributes narrowing, forcedFeatures excluded from cache key, singleton no-cross-pollution, SSE flush, missing/empty savedGroups, async LRU eviction, inflight coalescing, rule.tracks firing (including malformed-entry resilience)

Out of scope (deliberately)

• UserContext.forced_features on the async client — always sends []; add when a customer asks
• Encrypted remote-eval responses — JS SDK doesn't support this either
• Local AbstractStickyBucketService with remote eval — proxy handles sticky bucketing server-side; rejected at construction
• Running the benchmark harness against main — separate follow-up once the bench commit lands on main