Return the receipt for restored purchases

[timoschwarzer]

This PR allows getting the receipt for restored purchases. This is necessary if you want to verify a restored purchase on a server.

[naithar]

Thanks!

[Difference]

@timoschwarzer I tried to use a nuget package for reciept verification, but the there is a bug in Godot ios export that does not copy the dll. Do you have a way to parse the receipt?

Also: The appstore plugin should probably have GetReceipt(), RefreshReceipt() and maybe ValidateReceipt() as seperate functions, in order to get the original purchase date/version, when changing an app from Paid to Freemium.

Some more context here: godotengine/godot-docs#5197 (comment)

[Difference]

Made some C# code that sends the receipt to Apple servers, returning json. godotengine/godot-docs#5197 (comment)