Feature Request: Firebase SDK Auto Config

I would like to propose Firebase SDK AutoConfig as an option when initializing the app:
`AngularFireModule.initializeAppWithAutoConfig(),`

Right now, I have a workaround by adding this 2 lines in `package.json` so that the build process will pull the config from my Firebase project:
```
"build": "npm run getFirebaseConfig && ng build",
"getFirebaseConfig": "firebase apps:sdkconfig web --json > ./firebaseConfig.json",
```
Then in my `app.module.ts`:
```
import * as firebaseConfig from '../../firebaseConfig.json';
AngularFireModule.initializeApp(firebaseConfig.result.sdkConfig),
```
It works but it has an extra immediate step to generate the `firebaseConfig.json`. So, it will be great if AngularFireModule takes care of it.

Reference:
https://firebase.google.com/docs/hosting/reserved-urls#sdk_auto-configuration

Comece pela referência de autoconfiguração do SDK do Firebase para reserved-URLs e pela chamada AngularFireModule.initializeApp mostrada em app.module.ts. Compare isso com o workaround de package.json para build/getFirebaseConfig e com o fluxo de firebaseConfig.json; considera-se concluído quando a inicialização puder usar a autoconfiguração do SDK do Firebase sem esse arquivo gerado ou uma etapa extra de build.

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