Allow for static sms code

Currently the emulator returns a random String code when authenticating with phone against it. You can use this code to authenticate.

However for testing it would be much easier if the code was predictable, either via a configuration option or just a static code usage.

I've submitted a PR https://github.com/firebase/firebase-tools/pull/4255 that makes the code to always be '123456' but I do not know if that's the best option here.