[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.

Start by reviewing the existing firebase-admin/firestore entry point and comparing it with the frontend Firebase modular API described in the issue. The work is done when the Admin SDK supports the requested modular imports and server and frontend code can be shared as intended.

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