Enable automatic async/await API generation

[paulb777]

The GSI completion handlers use callback as the parameter selector for specifying completion handlers.

callback is not one of the selectors included for automatic async method generation. See https://developer.apple.com/documentation/swift/calling_objective-c_apis_asynchronously

If I manually change the selector in the GIDSignIn.h public header to completion, Xcode will auto-generate the API:

[peterfriese]

See Calling Objective-C APIs Asynchronously | Apple Developer Documentation for details. Specifically, if the trailing completion block uses the following names, it will automatically be made available to Swift as an async method:

• completion
• withCompletion
• completionHandler
• withCompletionHandler
• completionBlock
• withCompletionBlock
• replyTowithReplyTo
• reply
• replyTo

This is based on SE 0297, which discusses a number of Objective-C attributes that can be used to guide the translation process.