Add support for Apple Silicon

-Update classes to read correct Apple Silicon CPU sensor
-Update project to compile a Universal 2 binary. Upgraded Sparkle to 2.1.0 as that was the latest and supports ARM64.
-Add MacBookPro18,1 to Machines.plist

Sparkle.framework/Versions/B/PrivateHeaders/SUAppcastItem+Private.h

@@ -0,0 +1,45 @@
+//
+//  SUAppcastItem+Private.h
+//  Sparkle
+//
+//  Created by Mayur Pawashe on 4/30/21.
+//  Copyright © 2021 Sparkle Project. All rights reserved.
+//
+
+#ifndef SUAppcastItem_Private_h
+#define SUAppcastItem_Private_h
+
+#if __has_feature(modules)
+#if __has_warning("-Watimport-in-framework-header")
+#pragma clang diagnostic ignored "-Watimport-in-framework-header"
+#endif
+@import Foundation;
+#else
+#import <Foundation/Foundation.h>
+#endif
+
+NS_ASSUME_NONNULL_BEGIN
+
+// Available in SPUAppcastItemStateResolver.h (a private exposed header)
+@class SPUAppcastItemStateResolver;
+
+@interface SUAppcastItem (Private) <NSSecureCoding>
+
+/**
+ Initializes with data from a dictionary provided by the RSS class and state resolver
+
+ This initializer method is intended to be marked "private" and discouraged from public usage.
+ This method is available however. Talk to us to describe your use case and if you need to construct appcast items yourself.
+ */
+- (nullable instancetype)initWithDictionary:(NSDictionary *)dict relativeToURL:(NSURL * _Nullable)appcastURL stateResolver:(SPUAppcastItemStateResolver *)stateResolver failureReason:(NSString * _Nullable __autoreleasing *_Nullable)error;
+
+/**
+ The DSA and EdDSA signatures along with their statuses.
+ */
+@property (readonly, nullable) SUSignatures *signatures;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif /* SUAppcastItem_Private_h */