[Firestore] Typescript error on date

My environment

* Operating System version: macOS Monterey
* Firebase SDK version: 11.2.0
* Firebase Product: Firestore
* Node.js version: v16.17.1
* NPM version: 8.19.2

#### Steps to reproduce:

Just request a document with a timestamp, and get the seconds on the object:

```js
data.entryDate.seconds
```

and I get this typescript error:

```js
Any
Property 'seconds' does not exist on type 'string'.
Translation: You're trying to access seconds on an object that doesn't contain it.
```

使用包含时间戳的 Firestore 文档，在 Node.js 上通过 Firebase Admin SDK 11.2.0 重现该问题，然后检查 data.entryDate 的推断类型。将该类型与运行时对象进行比较，并验证修复完成后 TypeScript 能正确接受对 data.entryDate.seconds 的访问。

由索引模型根据 Issue 内容生成。