[in_app_purchase] Implement registerPlatform method for iOS implementation

[mvanbeusekom]

Implements the registerPlatform method in the InAppPurchaseIosPlatform class as discussed offline with @cyanglaz. This will allow callers to more easily register the InAppPurchaseIosPlatform as the default platform with InAppPurchasePlatform.

Also added a TODO comment to the README.md to remind us to add an example app.

This PR is part of the effort to migrate the in_app_purchase plugin to the Federated plugin architecture: flutter/flutter#78525

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. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• 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.
  • This change will be part of version 0.1.0 which is already merged into master but not yet published.
• I updated CHANGELOG.md to add a description of the change.
  • This change will be part of version 0.1.0 which is already merged into master but not yet published.
• 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.

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