[FR] Bundle size

**Is your feature request related to a problem? Please describe.**
When using `firebase-admin` on AWS lambda it adds a big blog to the bundle size. We are using `esbuild` to bundle our sources written in TS, and bundling the admin sdk adds an extra of 5.7MB, minified 2.8MB. This is far from optimal, because the bundle should be as small as possible for better cold-start performance. The code looks like this:

```js
import * as admin from "firebase-admin";

const app = admin.initializeApp();
console.log("app :>> ", app);

const firestore = app.firestore();
console.log("firestore :>> ", firestore);
```

**Describe the solution you'd like**
Make it possible to load or install only the parts of the SDK that are really needed (like the new version of the js client sdk => https://github.com/firebase/firebase-js-sdk)

**Describe alternatives you've considered**
I tried to add only parts of SDK we need, e.g.

```js
import { initializeApp } from "firebase-admin/lib/firebase-namespace";

const app = initializeApp();
console.log("app :>> ", app);

const firestore = app.firestore();
console.log("firestore :>> ", firestore);
```

but this gives the same bundle size and it does not work, even when compiling with `tsc`: When running via `node main.js` it gives this:

```bash
/some/path/main.js:6
var app = (0, firebase_namespace_1.initializeApp)();
^

TypeError: (0 , firebase_namespace_1.initializeApp) is not a function
```

Beginne mit dem TypeScript-Beispiel, das firebase-admin und den alternativen Einstiegspunkt firebase-admin/lib/firebase-namespace importiert, reproduziere dann das gemeldete Bundle mit esbuild und untersuche die generierte main.js. Die Aufgabe ist abgeschlossen, wenn das SDK nur die benötigten Teile laden kann und das resultierende Bundle deutlich kleiner ist, ohne die Initialisierung oder die Verwendung von Firestore zu beeinträchtigen.

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