[FR]: Why require google-services.json, if possible without it?

### Description

Right now it's pain in the butt to enable one project to work with multiple Firebase environments.

To avoid doing stupid stuff like creating/deleting google-services.json files with pre-build scripts, we just use the AppOptions constructor and manually provide all of the data google-services.json contains.
But iOS has annoying errors about plist missing, which forces us to keep google-services.json mock files for iOS and Android (with fake credentials).

Considering how non-optimal the file configuration UX is, why not make it optional?

P.S.If AppOptions is incomplete, would be great if you make it a complete alternative to config files.

### API Proposal

_No response_

### Firebase Product(s)

All

### Targeted Platform(s)

Apple Platforms, Android

Start by tracing the AppOptions constructor and the google-services.json and iOS plist configuration paths for the Unity SDK on Android and Apple platforms. Determine the scope of making those files optional and what a complete AppOptions alternative would require; done should be defined for all targeted Firebase products and both platforms.

Written by the indexing model from the issue text.