add-firebase-auth-bean.java

@Configuration
public class FirebaseAuthConfig {

    @Value("classpath:service-account.json")
    Resource serviceAccount;

    @Bean
    FirebaseAuth firebaseAuth() throws IOException {
        var options = FirebaseOptions.builder()
                .setCredentials(GoogleCredentials.fromStream(serviceAccount.getInputStream()))
                .build();

        var firebaseApp = FirebaseApp.initializeApp(options);

        return FirebaseAuth.getInstance(firebaseApp);
    }
}