Add App Check support for macOS #602

Is your feature request related to a problem you're having? Please describe.
App Check is currently not supported on macOS. The GIDSignIn.sharedInstance.configure method, which is required to enable App Check, is only available on iOS.

Looking at the source code, App Check related code is wrapped in #if TARGET_OS_IOS && !TARGET_OS_MACCATALYST, which confirms that macOS is intentionally excluded from the current implementation.

Describe the solution you'd like
Please add App Check support for macOS so that GIDSignIn.sharedInstance.configure (or an equivalent method) can be called on macOS as well.

Describe alternatives you've considered
No alternatives have been found. macOS apps are currently forced to run without App Check.

Additional context
Relevant source file: GoogleSignIn/Sources/GIDSignIn.m

#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
#import <AppCheckCore/GACAppCheckToken.h>
#import "GoogleSignIn/Sources/GIDAppCheck/Implementations/GIDAppCheck.h"
// ...
#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST