feat: Add getFeatureFlagResult API

[dustinbyrne]

💡 Motivation and Context

Add a getFeatureFlagResult API that returns a unified PostHogFeatureFlagResult object containing the flag key, enabled state, variant (for multivariate flags), and payload in a single call. Previously, getting full flag information required separate calls to getFeatureFlag and getFeatureFlagPayload. This also supports a sendEvent parameter to suppress the $feature_flag_called event when only the payload is needed.

getFeatureFlagPayload is now deprecated in favor of this new method.

💚 How did you test it?

• Added unit tests for PostHogFeatureFlagResult (fromValueAndPayload, fromMap, equality, toString, null/edge cases)
• Added integration tests via a fake platform interface covering boolean flags, multivariate flags, payloads, null values, and sendEvent parameter forwarding
• Updated the example app to exercise getFeatureFlagResult
• Manually verified native Android and iOS implementations serialize the result map correctly via method channels

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[dustinbyrne]

CI will fail until PostHog/posthog-ios#459 is released