Handle authorization response. #413

Context: After processing the authorization response by checking if there is an authorization code, we will want to handle the response by passing back a GIDVerifiedAccountDetailsResult to encapsulate the completion of the VwG flow and pass back the necessary tokens to the requesting party.

TODO:

1. Add properties to GIDVerifiableAccountDetailsResult.h
2. Create and implement method -[GIDVerifyAccountDetail addCompletionCallback:].
3. Unit testing for response handling.