feat: Add get_feature_flags_result method to client

[dustinbyrne]

Summary

• Add get_feature_flag_result method that returns a FeatureFlagResult object containing enabled state, variant, and payload in a single call
• Deprecate get_feature_flag_payload in favor of the new unified method
• Optimize feature flag lookup in get_feature_flag from O(n) to O(1) by using the existing @feature_flags_by_key hash

Motivation

Previously, getting a flag's value and payload required two separate calls:

value = client.get_feature_flag('my-flag', 'user-123')
payload = client.get_feature_flag_payload('my-flag', 'user-123')

The new method returns both in one call:

result = client.get_feature_flag_result('my-flag', 'user-123')
result.enabled?  # => true
result.variant   # => 'control' (or nil for boolean flags)
result.payload   # => { 'discount' => 10 }
result.value     # => variant if present, otherwise enabled

Related to PostHog/posthog#43520

[dmarticus]

Generally great; Ruby is a fun language! I liked the factory method and FeatureFlagResult design in general, and the O(n) -> O(1) optimization is a nice bonus.