[in_app_purchase_storekit] Add identifier, type fields to SKProductDiscountWrapper

[cyanglaz]

SKProductDiscount added "identifier", "type" two fields in iOS12.2, but the plugin is missing.
In some discount purchase scenarios, we need these fields.

Fixes flutter/flutter#100602

This PR is a fork of #5102
On top of the original PR, the version is updated to 0.3.2 instead of 0.3.1.

See #5102 (comment) for why the original PR is forked.

cc @fengyuxx

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 [relevant style guides] and ran [the auto-formatter]. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart 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], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[jmagman]

The native side LGTM, I'm not really familiar how the dart wrappers work though.

[jmagman]

@cyanglaz points out this was already reviewed in #5102, this LGTM.