Add signing certification and provisioning profile to GitHub workflow

We'd like to archive the `DaysUntilBirthday` sample app on a regular basis to ensure that clients depending upon GoogleSignIn-iOS will not be broken. Doing so requires signing the sample app. Here is some information on [this subject](https://docs.github.com/en/actions/deployment/deploying-xcode-applications/installing-an-apple-certificate-on-macos-runners-for-xcode-development).