[in_app_purchase_storekit] Add identifier, type fields to SKProductDiscountWrapper

[fengyuxx]

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

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/plugins repo does use dart format.)
• [x] I signed the [CLA].
• [x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed 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.

[flutter-dashboard[bot]]

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[stuartmorgan]

Thanks for the submission! @mvanbeusekom could you do the initial review here?

[jmagman]

Looks like this also needs a rebase onto the in_app_purchase_storekit changes on top of tree.

[jmagman]

@fengyuxx friendly bump, @cyanglaz had some requested changes on this PR. @mvanbeusekom were your requested changes addressed?

[stuartmorgan]

@cyanglaz Looks like this is ready for re-review.

[cyanglaz]

@fengyuxx Looks like there are still conflicts. Do you mind merging/rebasing upstream and resolve the conflicts?

[jmagman]

@fengyuxx This is ready to merge, would you mind fixing up the version and CHANGELOG conflict?

[stuartmorgan]

@cyanglaz Do you want to fork this PR into a new PR that we can land, since we're unable to fix the conflicts due to the permission restrictions on this branch?

[cyanglaz]

Forked at https://github.com/flutter/plugins/pull/6205

[stuartmorgan]

Closing as obsolete; https://github.com/flutter/plugins/pull/6205 landed.