Add support for keychain access groups

[rundfunk47]

This PR adds support for keychain access group as a parameter when initializing an instance. This is most usable when using Heimdall with app extensions, for example: if one would have a messaging client where push notifications are bundled with data which is encrypted according to a key generated and saved by the "main app", an app extension could be used to decrypt the data using the same key before presenting the push notification.