Running with Mac Catalyst doesn't launch browser to sign in.

Hi!

I'm trying to get Google sign-in to work with an iOS app + Mac Catalyst, but the Safari browser never launches on macOS in order to actually perform the sign-in. The same project works fine on an iPad simulator.

**Modified Sample Project**
I have modified the sample project to try this out: https://github.com/LovebugLLC/GoogleSignIn-iOS/commit/c5ca41ffc14fdb978d24175cdda88fc524655dd4

**Steps to reproduce my Sample Project edits**
1. Clone GoogleSignIn-iOS project
2. `open Samples/ObjC/SignInSample/SignInSample.xcodeproj`
3. Wait for Xcode to install dependencies
4. Select "SignInSample" target
5. Go to the "General" tab
6. Add Mac Catalyst app
7. Go to the "Signing & Capabilities" tab
8. Select your Development Team
9. Click "+ Capability", and add "Keychain Sharing"
10. Run scheme `SignInSample -> My Mac (Catalyst)`

**Steps to reproduce the issue, after running the app**
1. Tap "Sign in" and you'll see this screen:
2. Click "Continue"

**Expected results**
Browser launches so I can log in

**Actual results**
No browser shows up

**Other notes**
If I change the run target to an iPad simulator, I am presented with the browser window and login just fine.