Add support for container groups

[mikezliu]

When the settings plist contains the string ApplicationGroupContainerIdentifier and a custom container name, it's not currently used by InAppSettingsKit. See https://developer.apple.com/library/content/documentation/PreferenceSettings/Conceptual/SettingsApplicationSchemaReference/Articles/RootContent.html for information about this key. Container groups are used for sharing settings such as between the app and its widgets.

Looking in IASKSettingsStoreUserDefaults, it appears that standardUserDefaults is always used. This should call initWithSuiteName with the name of the container from the key ApplicationGroupContainerIdentifier.

[futuretap]

Thanks for noticing this. Indeed, we don't support the ApplicationGroupContainerIdentifier yet. Would you be willing to contribute a fix in a pull request?