Feat: Add keychainAccessGroup configuration option

[thorrak]

This PR adds an optional configuration option keychainAccessGroup which sets the keychain-access-groups entitlement on both the main app and the widget during prebuild.

This entitlement is similar to #7 in both purpose and implementation, but while App Groups are typically used to insecurely share key/value pairs between an app and a widget, keychain-access-groups allow you to use packages like expo-secure-store to securely share values via the iOS Keychain.

Here's an example of implementing this in Swift:

import Security

// Function to retrieve values from expo-secure-store (iOS Keychain)
func getSecureStoreValue(forKey key: String) -> String? {
    let query: [String: Any] = [
        kSecClass as String: kSecClassGenericPassword,
        kSecAttrAccessGroup as String: "<KEYCHAIN ACCESS GROUP ID HERE>",
        kSecAttrAccount as String: key,
        kSecReturnData as String: true,
        kSecMatchLimit as String: kSecMatchLimitOne
    ]

    var item: CFTypeRef?
    let status = SecItemCopyMatching(query as CFDictionary, &item)

    guard status == errSecSuccess,
          let data = item as? Data,
          let value = String(data: data, encoding: .utf8)
    else { 
        print("Widget: Failed to retrieve secure store value for key '\(key)'. Status: \(status)")
        return nil 
    }

    print("Widget: Successfully retrieved secure store value for key '\(key)'")
    return value
}

And implementing writing the item to the keychain in React Native:

import * as SecureStore from 'expo-secure-store';
const TOKEN_KEY = "KeyToSave";
const GROUP = '<KEYCHAIN ACCESS GROUP ID HERE>';

export const saveToken = async (token: string): Promise<void> => {
  await SecureStore.setItemAsync(TOKEN_KEY, token, {
    accessGroup: GROUP,
    keychainAccessible: SecureStore.AFTER_FIRST_UNLOCK
  });
};

export const getToken = async (): Promise<string | null> => {
  return await SecureStore.getItemAsync(TOKEN_KEY, {
    accessGroup: GROUP,
    keychainAccessible: SecureStore.AFTER_FIRST_UNLOCK
  });
};

export const removeToken = async (): Promise<void> => {
  await SecureStore.deleteItemAsync(TOKEN_KEY, {
    accessGroup: GROUP
  });
};

[nathan-ahn]

Thank you for your PR :)