Remove Authentication Requirement for Subscription Flow

[Piinks]

Original: flutter/flutter#122136

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Use case

Apple has rejected iOS submissions for apps leveraging the Flutter News Toolkit based on required user authentication for the subscription flow. Per Apple Guideline 5.1.1, apps cannot require user registration prior to allowing access to app content and features that are not associated specifically to the user. User registration that requires the sharing of personal information must be optional or tied to account-specific functionality.

Per Apple, although App Store Review Guideline 3.1.2 requires an app to make subscription content available to all the iOS devices owned by a single user, it is not appropriate to force user registration to meet this requirement; such user registration must be optional.

Proposal

I propose we modify the logic in the toolkit to support optional authentication for subscriptions. Users should be prompted with a login option to access their subscription content across all devices and optionally opt out and proceed through the in-app purchasing flow. Here's an example prompt that could be leveraged:

This is change should be made by Flutter or a contributor, so users of the toolkit can take advantage of this change across the board.