Implement feature flag cache and flag-called dedupe

[marandaneto]

Summary

Implement canonical feature flag cache behavior and $feature_flag_called dedupe by flag value for the Unity SDK.

Expected behavior

Feature flag cache

• Successful flag loads cache flag values and payloads.
• Cached reads do not require another network request.
• Cached values/payloads are available to the public flag getter APIs.
• Cache is cleared when identity reset invalidates user-scoped flags.

$feature_flag_called dedupe

• Emit the first $feature_flag_called event for a flag value.
• Suppress duplicate events for the same flag and same value.
• Emit again when the flag value changes.

Specs

• https://github.com/PostHog/sdk-specs/blob/main/acceptance/private/feature-flag-cache.feature
• https://github.com/PostHog/sdk-specs/blob/main/acceptance/private/feature-flag-called-tracker.feature