Add `-[GIDSignIn signInWithCompletion:]` to provide sign in mechanism without a `presentingViewController`

`GIDSignInInternalOptions`, depending upon whether or not we are on macOS or iOS, will need to be created with a nil `presentingWindow` or `presentingViewController`.

The sign in with hint methods will also need to be updated similarly as well.

```
-[GIDSignIn signInWithHint:completion:]
-[GIDSignIn signInWithHint:addAdditionalScopes:completion:]
```

Last, the `-authenticateInteractivelyWithOptions:` method needs to be updated to handle the nil window or view controller case. The new implementation will make use of the new `+[OIDAuthorizationService presentAuthorizationRequest:callback:]` method outlined in the [related milestone on AppAuth-iOS](https://github.com/openid/AppAuth-iOS/milestone/7).