[in_app_purchase_storekit] Add support to win back offers / promotional offers

[kaiquegazola]

Adds StoreKit2 support for:

• Win Back Offers (requires Xcode 16 or later)
  • New checkWinBackOfferEligibility method for eligibility checks
• Promotional Offers
• Fixes introductory pricing handling in promotional offers list
• Fixes appAccountToken handling for StoreKit2 purchases

Fixes:
flutter/flutter#161393
flutter/flutter#160826

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene 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/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to 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, or this PR is exempt from CHANGELOG changes.
• 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.

[LouiseHsu]

Hey just letting you know Ive looked at your PR, and it looks really good! I've checked out your branch and it seems to work well locally. However our tests are failing to compile because currently they're running on ios 17.0. I'm taking a look to see if theres a way around this, but if you want to try building your branch on Xcode 15 or lower that would be really helpful too!

[kaiquegazola]

Thanks for for testing it locally

It seems we’re currently blocked by flutter/flutter#148870. Also, I think we should add a CHANGELOG warning regarding this Xcode requirement.

Maybe an #if compiler directive could help here, but I haven’t tested it yet, so I’m not sure if it actually works. Let me know what you think

[kaiquegazola]

Hi @LouiseHsu, when you have a moment, could you please take a look at this PR? Thanks