Sign In with Apple: check login status

*.swift

        let userIdentifier = // get saved userIdentifier - sample app uses Keychain
        appleIDProvider.getCredentialState(forUserID: userIdentifier) { (credentialState, error) in
            switch credentialState {
            case .authorized:
                // The Apple ID credential is valid.
                break
            case .revoked:
                // The Apple ID credential is revoked.
                break
            case .notFound:
                // No credential was found, so show the sign-in UI.
                // code ....
            default:
                break
            }
        }