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 individual experimental features (instead of wholesale via 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. As documented in the README, this is absolutely a use-at-your-own-risk config plugin that developers should apply carefully.

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 separate RNFeatureFlagsOverrideProvider.mm module, imports via the bridging header, and inserts a call into AppDelegate.swift ahead of factory.startReactNative().
• Android: Inserts imports and an override call into MainApplication.kt after loadReactNative(this) (SoLoader must be initialized first).

Test Plan

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