[in_app_purchase_storekit] migrate main plugin class to swift in preperation to storekit 2 (flutter#6561)

Part of flutter#119106

This PR migrates the main InAppPurchasePlugin.m class to swift.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [relevant style guides] and ran the
auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages
repo does use `dart format`.)
- [x] I signed the [CLA].
- [x] The title of the PR starts with the name of the package surrounded
by square brackets, e.g. `[shared_preferences]`
- [x] I [linked to at least one issue that this PR fixes] in the
description above.
- [x] I updated `pubspec.yaml` with an appropriate new version according
to the [pub versioning philosophy], or this PR is [exempt from version
changes].
- [x] I updated `CHANGELOG.md` to add a description of the change,
[following repository CHANGELOG style].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

Author: LouiseHsu

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.16
+
+* Converts main plugin class to Swift
+
 ## 0.3.15
 
 * Replaces `getCountryCode` with `countryCode`.