[auth-swift] Multi Platform Sample

[paulb777]

Create a SwiftUI Sample for FirebaseAuth initially building for iOS, macOS and visionOS

It currently depends on forks of GoogleSignIn and AppAuth for visionOS support

The visionOS flow works correctly.

TODO: Investigate why the IOS sign in screen is blank and the macOS flow hangs with a permission error after signing in.

#no-changelog

[paulb777]

macos failure:

Error doing Google Sign-In, Error Domain=org.openid.appauth.general Code=-5 "Connection error making token request to 'https://oauth2.googleapis.com/token': A server with the specified hostname could not be found.." UserInfo={NSLocalizedDescription=Connection error making token request to 'https://oauth2.googleapis.com/token': A server with the specified hostname could not be found.., NSUnderlyingError=0x600000bcb7e0 {Error Domain=NSURLErrorDomain Code=-1003 "A server with the specified hostname could not be found."