Added grace period fields

[jun06t]

Support grace period fields.

If you choose to enable Billing Grace Period, ensure that you provide full service for the subscription throughout the grace period. You can check the grace_period_expires_date_ms field in the responseBody.Pending_renewal_info array of the receipt to determine the end of this grace period duration.

ref: https://developer.apple.com/documentation/storekit/in-app_purchase/reducing_involuntary_subscriber_churn?language=swift

Jacob already confirmed that the receipt has those new fields.

• grace_period_expires_date
• grace_period_expires_date_ms
• grace_period_expires_date_pst

ref: https://www.revenuecat.com/2019/09/13/supporting-ios-billing-grace-period

[codecov]

Codecov Report

Merging #96 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   90.36%   90.36%           
=======================================
  Files           4        4           
  Lines         249      249           
=======================================
  Hits          225      225           
  Misses         16       16           
  Partials        8        8

Impacted Files	Coverage Δ
appstore/model.go	100% <ø> (ø)	⬆️

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[kitakitabauer]

LGTM

[jun06t]

Thanks @kitakitabauer :-)