This PR introduces a new persistence plugin that enables secure storage of observable state in React Native applications using the device keychain, based on react-native-keychain package

Changes

• Added react-native-keychain dependency - Required for secure keychain access on iOS and Android
• Implemented observablePersistKeychain plugin (src/persist-plugins/keychain.ts) - A complete persistence plugin with full CRUD operations for keychain storage
• Added comprehensive test suite
• Updated sync types - Added necessary type definitions for the new plugin

Features

• ✅ Secure storage using device keychain
• ✅ Graceful error handling with customizable error callbacks
• ✅ Preloading capabilities

Usage Example

import { observablePersistKeychain } from '@legendapp/state/persist-plugins/keychain';
import { configureSynced } from '@legendapp/state/sync';

const mySynced = configureSynced(synced, {
    persist: {
        plugin: observablePersistKeychain(),
    },
});

const userState = observable({
    token: '',
    preferences: { theme: 'dark' }
});

const state = syncObservable(
    userState,
    mySynced({
        persist: { name: 'secure-user-data' },
    }),
);

This PR addresses issue #548