Implement hasReceipt method

[franvera]

Method is useful for knowing if there is a receipt or not. This way we can prevent prompting the user on start or when the app has not completed any purchases

[chirag04]

Code wise this looks good. Can you point me to some documentation regarding this?

[franvera]

@chirag04 well, you know how great Apple documentation is </sarcasm>.
Only official info I can find is here, where Apple points out that the receipt is not guaranteed to be present: https://developer.apple.com/documentation/foundation/nsbundle/1407276-appstorereceipturl
For what I have experienced, read in this repo and found on stackoverflow, it seems that new or restored apps have no valid receipt locally. hsaReceipt can be used to know if that's the scenario and ask user to either purchase/subscribe or restore purchases.