Add App Check support for macOS

**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`

```objc
#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
#import
#import "GoogleSignIn/Sources/GIDAppCheck/Implementations/GIDAppCheck.h"
// ...
#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST
```