Add `-[GIDSignIn addScopes:completion:]`to provide add scopes flow without a presenting view controller or window

The main goal of this method will be to create an instance of `GIDSignInInternalOptions` with no presenting window or view controller. Something like:

```
GIDSignInInternalOptions *options =
[GIDSignInInternalOptions defaultOptionsWithConfiguration:configuration
#if TARGET_OS_IOS || TARGET_OS_MACCATALYST
presentingViewController:nil
#elif TARGET_OS_OSX
presentingWindow:nil
#endif // TARGET_OS_IOS || TARGET_OS_MACCATALYST
loginHint:self.currentUser.profile.email
addScopesFlow:YES
completion:completion];
```