[AltStoreCore] Refactors core AltStore logic into AltStoreCore framework

AltStoreCore will contain all shared AltStore code between AltStore and any app extensions. Initially, it includes all AltStore model logic.

AltStoreCore/Model/SecureValueTransformer.swift

@@ -0,0 +1,26 @@
+//
+//  SecureValueTransformer.swift
+//  AltStore
+//
+//  Created by Riley Testut on 8/18/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import Foundation
+
+@objc(ALTSecureValueTransformer)
+public final class SecureValueTransformer: NSSecureUnarchiveFromDataTransformer
+{
+    public static let name = NSValueTransformerName(rawValue: "ALTSecureValueTransformer")
+    
+    public override static var allowedTopLevelClasses: [AnyClass] {
+        let allowedClasses = super.allowedTopLevelClasses + [NSError.self]
+        return allowedClasses
+    }
+    
+    public static func register()
+    {
+        let transformer = SecureValueTransformer()
+        ValueTransformer.setValueTransformer(transformer, forName: name)
+    }
+}