Make `SubscriptionType` Hashable.

Sources/SwiftyStoreKit/SwiftyStoreKit+Types.swift

@@ -175,7 +175,7 @@ public enum VerifySubscriptionResult {
     case notPurchased
 }
 
-public enum SubscriptionType {
+public enum SubscriptionType: Hashable {
     case autoRenewable
     case nonRenewing(validDuration: TimeInterval)
 }