[feat] could we use FirebaseApp.initializeApp manually at workthread to reduce the startup time cost？

## What feature would you like to see?

Describe the feature you would like to add, ideally proposing a specific API.

the FirebaseApp.initializeApp will cost about 60ms on my app startup stage, could we could we use FirebaseApp.initializeApp manually at workthread to reduce the startup time cost？is there any problems that init it at work thread?

thanks~
## How would you use it?

Tell us how you'd use this feature in your app.