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

[kaiquegazola]

Adds support for Win Back Offers and Promotional Offers in StoreKit2.
Fixes introductory pricing handling in promotional offers list.
Fixes appAccountToken handling in StoreKit2 purchases.

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

Pre-launch Checklist

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