feat: Add getFeatureFlagResult method to client

[dustinbyrne]

Adds getFeatureFlagResult() method that returns both the feature flag value and payload in a single call while emitting $feature_flag_called.

Related to PostHog/posthog#43520

Changes

• New FeatureFlagResult class with methods:

  • isEnabled() - whether the flag is enabled
  • getVariant() - the variant string for multivariate flags
  • getPayload() - the decoded JSON payload
  • getValue() - returns variant if set, otherwise the enabled boolean (matches getFeatureFlag() format)

• New getFeatureFlagResult() method on both Client and PostHog classes

  • Returns FeatureFlagResult|null
  • Sends $feature_flag_called event for usage tracking
  • Supports all existing parameters (groups, personProperties, onlyEvaluateLocally, etc.)

• Refactored getFeatureFlag() to delegate to getFeatureFlagResult() internally

• Deprecated getFeatureFlagPayload() - it doesn't send the $feature_flag_called event, leading to missing usage analytics and confusion. Users should migrate to getFeatureFlagResult().

[dmarticus]

also clean