[FR] Modular Import Support for Firebase Admin SDK

**Is your feature request related to a problem? Please describe.**
The current Firebase Admin JS SDK does not support modular imports similar to the frontend Firebase SDK. For example, in the frontend SDK, I can use modular imports like:

`import { doc, getDoc } from "firebase/firestore";`

**Describe the solution you'd like**
I would like the Firebase Admin SDK to support modular imports similar to the frontend SDK. For example:

`import { doc, getDoc } from "firebase-admin/firestore";`

This would align the Admin SDK with the frontend SDK’s modular API and provide a more consistent developer experience.

**Describe alternatives you've considered**
Creating a custom wrapper around the Admin SDK to mimic modular imports. However, this adds unnecessary complexity and maintenance overhead.

**Additional context**
We'd like to share code between server and frontend.