Add a privacy manifest for XCode 15

[lindboe]

Starting with Xcode 15, Apple is requiring that libraries and apps that use specific features register their “reason” for doing so in a new privacy manifest. This includes UserDefaults, which react-native-share-menu uses to store shared data to pass to the main app. Its usage of UserDefaults is fully compliant with the allowed “reason” (“Declare this reason to access user defaults to read and write information that is only accessible to the app itself”), so we only need to register the reason and we’re done.

How to create a privacy manifest: https://developer.apple.com/documentation/bundleresources/privacy_manifest_files