Process authorization response and token fetch. #405

Context: After receiving the authorization request, the next step is to process the authorization response. This will follow GIDSignIn's current implementation: -[GIDSignIn processAuthorizationResponse:error:emmSupport:]. This method will check for an authorization code. If it does then it fetches the age access token, otherwise an error will be issued to the client.

TODO:

1. Implement -[GIDVerifyAccountDetail processAuthorizationResponse:error:emmSupport:].
2. Fetch token similar to -[GIDSignIn maybeFetchToken].
3. Unit testing for request and response handling.