[Auth] Add @discardableResult to avoid warnings

[ns-vasilev]

This PR adds @discardableResult to avoid warnings, as described in #13649

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[paulb777]

Thanks @ns-vasilev. Please sign the CLA and we can take a look.

[ns-vasilev]

@paulb777 I've done this

[paulb777]

@ns-vasilev See https://github.com/firebase/firebase-ios-sdk/pull/14486/checks?check_run_id=37718450372

[ns-vasilev]

@paulb777, I didn't notice that it's possible to restart the check, thanks 👍

[ncooke3]

Thanks @ns-vasilev for exploring both approaches! New APIs follow an internal API proposal process so I will kick that off and circle back with any feedback