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

从 Firebase reserved-URLs SDK 自动配置参考文档以及 app.module.ts 中显示的 AngularFireModule.initializeApp 调用开始。将其与 package.json 中的 build/getFirebaseConfig workaround 和 firebaseConfig.json 流程进行比较；完成的标准是初始化可以使用 Firebase SDK 自动配置，而不需要该生成的文件或额外的 build 步骤。

由索引模型根据 Issue 内容生成。