File picker and NSPhotoLibraryUsageDescription permission on iOS

[FeodorFitsner]

https://github.com/miguelpruivo/flutter_file_picker/pull/985

ITMS-90683: Missing Purpose String in Info.plist - Your app‘s code references one or more APIs that access sensitive user data. The app‘s Info.plist file should contain a NSPhotoLibraryUsageDescription key with a user-facing purpose string explaining clearly and completely why your app needs the data. If you're using external libraries or SDKs, they may reference APIs that require a purpose string. While your app might not use these APIs, a purpose string is still required. For details, visit: https://developer.apple.com/documentation/uikit/protecting_the_user_s_privacy/requesting_access_to_protected_resources