feat: Add react-native-feature-flags plugin (SDK 54+)

[huntie]

Why

New config plugin to allow override customisation of ReactNativeFeatureFlags on iOS and Android.

This is intended for advanced users, to opt into experimental features ahead of a release level, or to disable features that conflict with a specific app setup.

⚠️ Important: Features controlled by these low level flags are always experimental and unstable.

How

• Flag names map 1:1 to method names on the base provider class. A compile-time error will surface if a flag doesn't exist in the installed React Native version.
• Uses dangerouslyForceOverride() on both platforms, since RCTReactNativeFactory (iOS) and Expo's Android setup already call override() during initialization.
• iOS: Generates .mm and .h files, adds them to the Xcode project, imports via the bridging header, and inserts a call into AppDelegate.swift.
• Android: Inserts imports and an override call into MainApplication.kt after super.onCreate() (SoLoader must be initialized first).

Test Plan

See added tests. End-to-end verified locally.