Expose and document the Firestore.projectId getter

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### Documentation Request

Easy access to `db.projectId` would help diagnose [situations in which it's not clear that the correct project ID was picked up](https://stackoverflow.com/questions/79336721/where-does-firebase-admin-sdk-firestore-pick-up-its-project-id-from-if-not-sp).

```ts
import { initializeApp } from 'firebase-admin/app';
import { getFirestore } from 'firebase-admin/firestore';

initializeApp();
const db = getFirestore();

// Running the run line below triggers "INTERNAL ERROR: Client is not yet ready to issue requests"
// console.log('Database project ID:', (db as unknown as { projectId: string})['projectId']);
```

It would also be useful to document *when* exactly the `projectId` becomes available (empirically after the first read operation).