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

從 Firebase reserved-URLs SDK 自動設定參考文件以及 app.module.ts 中顯示的 AngularFireModule.initializeApp 呼叫開始。將其與 package.json 中的 build/getFirebaseConfig workaround 和 firebaseConfig.json 流程進行比較；完成的標準是初始化可以使用 Firebase SDK 自動設定，而不需要該產生的檔案或額外的 build 步驟。

由索引模型根據 Issue 內容生成。