Add full onFeatureFlags callback with flags, variants, and error state

[ioannisj]

Description

Mobile currently lacks parameters in the ‎onFeatureFlags callback, causing inconsistency with Web. We use a void callback that only notifies when flags are loaded, but the intention is to pass evaluated flags, variants, and any error state.

Context: PostHog/posthog-flutter#224 (comment)

• iOS (this issue)
• Android (Add full onFeatureFlags callback with flags, variants, and error state posthog-android#563)
• Flutter (Add full onFeatureFlags callback with flags, variants, and error state posthog-flutter#231)
• RN