Clean up callback flow for sign-in and verify flow.

Issue found in: https://github.com/google/GoogleSignIn-iOS/pull/414#discussion_r1607303702

The current sign-in and verify flow are brittle when processing the authorization response. The current flow will depend on an `[authFlow wait]` to ensure a token request is made before moving forward. Doing so makes the code harder to understand and easy to break. A way to improve this is by adding completion callbacks to ensure a `tokenRequest` is made before returning an instance of `GIDAuthFlow.`