Update go-iap to sync with upstream

Syncs with fork upstream while maintaining local changes to make NumericString type public

Make numericString type exportable

Make the numericString type exportable that is in Enflick/go-iap/appstore/models.go

Add omitempty to original_transaction_id

Merge pull request #10 from Enflick/IAP-259_Add_omitempty

IAP-259: Add omitempty to the original transaction id

Changed another field

Merge pull request #9 from Enflick/IAP-259_More_Changes

IAP-259: One more place to change

Restore go mod and sum

Merge pull request #8 from Enflick/IAP-259

IAP-259: Restore files to previous state

Updated notification.go

Updated App Store notification and subscription with new type.