Improve app group configuration for iOS

[lindboe]

The library incorrectly assumes that all app groups must match the pattern group.<main.app.bundle.id> . This isn't necessarily the case for all apps. We’ll update the library to instead have the app group be directly configured in Info.plist, instead of configuring it with the host app’s bundle ID.

Scope:

• [ ] Allow app group ID to be configured directly in Info.plist and use that anywhere the app group is needed
• [ ] remove the host app bundle ID as config if it's no longer used (or alternatively, for backwards compatibility, allow it to be used to construct the group name, but ignore this value if app group ID is set in Info.plist)
• [ ] Create a changelog entry for this
• [ ] Update documentation to reflect this change and recommend use of the app group ID configuration