Feature/inject receipt validator

[daveluong]

Description

This PR serves as ground work to a patch to security issue where this framework calls Apple's receipt validation endpoints directly which is susceptive to MITM attack whereby attacker can spoof the receipt validation.

This PR also enables framework users to inject their own custom receipt validator. There are 3 scenarios where this is useful:

• Framework users who want to implement server-to-server receipt validation (their own backend - Apple's validation service)

• Framework users who want to handle validation on their own backend

• Client-side receipt validation implementation

Code related changes

• New ReceiptValidator protocol. Custom receipt validator should conform to this protocol

public protocol ReceiptValidator {
	func validate(receipt: String, password autoRenewPassword: String?, completion: @escaping (VerifyReceiptResult) -> Void)
}

• Modified signature of verifyReceipt function to let user to inject custom receipt validator object

• Extract existing validation code to AppleReceiptValidator that conforms to ReceiptValidator

• Updated examples to use AppleReceiptValidator, this should be served only for demonstration purpose, shouldn't use this validator for production code

[bizz84]

Thank you for this PR! I'll review it this weekend.

[bizz84]

Great work, thanks! Now available on pod version 0.6.2: 🚀

https://github.com/bizz84/SwiftyStoreKit/releases/tag/0.6.2

[daveluong]

Thank you so much! Great and useful library!