[in_app_purchase] Migrate plugin to federated architecture

[mvanbeusekom]

Use case

Migrate the "in_app_purchase" package to the federated plugin architecture. This would result in the following benefits:

1. Makes it easier to add support for additional platforms (there is already a concrete use case for macOS, but Linux and Windows are also potential candidates);
2. The federated architecture also makes it easier to write automated tests and guard the API;
3. Confirms to the architecture of most other Flutter plugins, making it easier for developers to understand the project layout and contribute to the project.

Proposal

To migrate the in_app_purchase package to federated architecture we should take the following steps:

• Move the in_app_purchase into its own subdirectory ("in_app_purchase/in_app_purchase");
• Create the in_app_purchase_platform_interface package;
  • Copy the LICENSE file from the in_app_purchase package;
  • Add a CHANGELOG.md file;
  • Define a pubspec.yaml file;
  • Create a README.md file;
  • Define the platform interface in in_app_purchase_platform.dart;
  • Implement the default implementation in method_channel_in_app_purchase.dart;
• Refactor package in_app_purchase to use the in_app_purchase platform interface;

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