[in_app_purchase] Implementation of platform interface

[mvanbeusekom]

Defines a common platform interface for the in_app_purchase plugin.

• Takes care of the in_app_platform_interface section of issue [in_app_purchase] Migrate plugin to federated architecture flutter#78525

NOTE:

As part of the in_app_purchase_platform_interface we will be replacing the following methods:

• the Future<BillingResultWrapper> completePurchase(PurchaseDetails) method is replaced by the Future<void> finishPurchase(PurchaseDetails) method. The new finishPurchase method will no longer return the Android specific BillingResultWrapper, instead it will complete the future successfully if the finishPurchase executed successfully or throw an instance of the PurchaseException when an error occurred. The PurchaseException should be implemented per platform to indicate if developers should retry based on platform specific error codes;
• the Future<QueryPurchaseDetailsResponse> queryPastPurchases({String? applicationUserName}) method is replaced by the Future<void> restorePurchases({String? applicationUserName}) method. The new restorePurchases will no longer return a list of restored purchases but instead emits each purchase on the new purchaseStream with the specific PurchaseStatus.restored status;
• and the Stream<List<PurchaseDetails>> get purchaseUpdatedStream is replaced by the Stream<List<PurchaseDetails>> get purchaseStream property. The new purchaseStream should no longer filter out purchases with the PurchaseStatus.restored status, but instead emit those as well, giving developers the change to validate the receipt, deliver the content and finish the purchase correctly.

The existing Android and iOS implementations should mark the above methods as deprecated and should provide implementation for the new methods.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[cyanglaz]

Discussed offline, we are going to add a InAppPurchaseExtend and InAppPurchaseExtendProvider to the platform interface so Platform specific code can extend features.

CC @stuartmorgan @mvanbeusekom
Please comment with better name for these 2 classes if you can think of any :)

[cyanglaz]

we should remove the iOS related doc here and make sure it is documented in the plugin.

[cyanglaz]

[consumePurchase] is not part of the platform interface, let's remove this line

[stuartmorgan]

Discussed offline, we are going to add a InAppPurchaseExtend and InAppPurchaseExtendProvider to the platform interface so Platform specific code can extend features.

CC @stuartmorgan @mvanbeusekom
Please comment with better name for these 2 classes if you can think of any :)

How about s/Extend/Additions/ (It would need to be Extension to be the right part of speech, but that's a potentially confusing term given Dart's extension methods)

[cyanglaz]

LGTM % some doc nits

[cyanglaz]

Let's also add doc for how to add additional features in a platform

[mvanbeusekom]

done

[cyanglaz]

Let's add a code example of how a platform can add "Addition".
Something like:

class InAppPurchaseSomeStoreAddition extends InAppPurchaseAddition {
    void someMethod() {}
}

class InAppPurchaseSomeStorePluginWeb {
    static void registerWith() {
         InAppPurchaseAddition = InAppPurchaseSomePlatformAddition();
     }
}

[mvanbeusekom]

Done

[cyanglaz]

The publishable check failed due to some tooling bug, which is fixed on master. Rebase should resolve the issue

[cyanglaz]

LGTM

[cyanglaz]

We will hold off publishing the platform interface until we integrated with the plugin and made sure everything works. Please keep the "needs-publishing" label on