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: #224 (comment)

• iOS
• Android
• Flutter
• react-native