initializing a firebase::App object with a path to the google-services.json file?

### Please fill in the following fields:
Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: pre-built
Firebase C++ SDK version: 6.14.0
Firebase plugins in use (Auth, Database, etc.): firestore
Additional SDKs you are using (Facebook, AdMob, etc.): none
Platform you are using the C++ SDK on (Mac, Windows, or Linux): linux
Platform you are targeting (iOS, Android, and/or desktop): desktop

### Please describe the issue here:
(Please list the full steps to reproduce the issue. Include device logs, Unity logs, and stack traces if available.)

Is there a option for initializing a firebase::App object with a path the the google-services.json file? It looks like all the code is available in the SDK, but hidden in an anonymous namespace. This is being used in a standard linux desktop environment with no build tools besides cmake.

### Please answer the following, if applicable:
Have you been able to reproduce this issue with just the [Firebase C++ quickstarts](https://github.com/firebase/quickstart-cpp) ? N/A

What's the issue repro rate? (eg 100%, 1/5 etc) N/A

Start at the firebase::App initialization entry point for Linux desktop and trace how google-services.json is currently handled in the pre-built SDK. The issue says the relevant code is hidden in an anonymous namespace and the environment has only CMake available. Done means establishing whether initialization can accept a JSON path and verifying the behavior on the stated platform.

Written by the indexing model from the issue text.