Supports blocking third-party sources

Blocked sources cannot be added by new users, or updated for existing users.

AltStoreCore/Extensions/UserDefaults+AltStore.swift

@@ -118,3 +118,40 @@ public extension UserDefaults
         }
     }
 }
+
+public extension UserDefaults
+{
+    // Cache trustedSourceIDs just in case we need to check whether source is trusted or not.
+    @nonobjc var trustedSourceIDs: Set<String>? {
+        get {
+            guard let sourceIDs = _trustedSourceIDs else { return nil }
+            return Set(sourceIDs)
+        }
+        set {
+            _trustedSourceIDs = newValue?.map { $0 }
+        }
+    }
+    @NSManaged @objc(trustedSourceIDs) private var _trustedSourceIDs: [String]?
+    
+    @nonobjc var blockedSourceIDs: Set<String>? {
+        get {
+            guard let sourceIDs = _blockedSourceIDs else { return nil }
+            return Set(sourceIDs)
+        }
+        set {
+            _blockedSourceIDs = newValue?.map { $0 }
+        }
+    }
+    @NSManaged @objc(blockedSourceIDs) private var _blockedSourceIDs: [String]?
+    
+    @nonobjc var blockedSourceURLs: Set<URL>? {
+        get {
+            guard let sourceURLs = _blockedSourceURLs?.compactMap({ URL(string: $0) }) else { return nil }
+            return Set(sourceURLs)
+        }
+        set {
+            _blockedSourceURLs = newValue?.map { $0.absoluteString }
+        }
+    }
+    @NSManaged @objc(blockedSourceURLs) private var _blockedSourceURLs: [String]?
+}