libraries: add @react-native-firebase/app

[Simek]

Why

One of top most used libraries according to the npm stats.

Refs:

• Introducing the React Native Nightly testing program 🌑🌓🌔🌕🌖🌗 discussions-and-proposals#931 (comment)

How

Add @react-native-firebase/app to the libraries list.

[cortinico]

@cipolleschi I believe mentioned that we need to edit the template here right?
I think we should have a /patches root folder where we optionally store a <library-name>.patch file that is applied to the project before building.