CWIZ-273 add custom ios options for auth storage

[kstrathdee-evoleen]

add option to provide custom ios options for auth storage

This pr adds the option to pass custom iOS options to the auth storage constructor.

This enables us to specify the level of keychain accessibility that we want for our token storage, which will enable the resolution of a bug where we are unable to fetch our cached token while the app is backgrounded.

upstream PR here: Earlybyte#373

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Note

Adds configurable iOS IOSOptions for secure token storage, wired via Config into AuthStorage and MobileOAuth.

• Auth Storage
  • AuthStorage now requires IOSOptions and passes it to FlutterSecureStorage (lib/helper/auth_storage.dart).
• Config
  • Introduces iOptions: IOSOptions with default IOSOptions.defaultOptions and exposes in copyWith (lib/model/config.dart).
• OAuth Wiring
  • MobileOAuth constructs AuthStorage with config.iOptions (lib/helper/mobile_oauth.dart).

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