[firebase_auth] token issued by createCustomToken are not valid with emulated in <<demo>> project

As per the documentation:

<< For security reasons, the Authentication emulator issues unsigned ID tokens, which are only accepted by other Firebase emulators, or the Firebase Admin SDK when [configured](https://firebase.google.com/docs/emulator-suite/connect_auth#admin_sdks). >>

So when there is an env variable `FIREBASE_AUTH_EMULATOR_HOST="127.0.0.1:9099"` firebase admin is supposed to give unsigned tokens via the emulator, which it does not:

The following:

```ts
const user = await admin.auth().createUser({ uid: randomUUID() });
const jwt = await admin.auth().createCustomToken(user.uid);
```

Results in:
```
Firebase ID token has incorrect "aud" (audience) claim. Expected "demo-test" but got "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit"
```

首先，在将 FIREBASE_AUTH_EMULATOR_HOST 设置为 127.0.0.1:9099 的情况下复现 TypeScript 示例，然后跟踪 createCustomToken 路径及其模拟器配置。当为 demo-test 项目创建的自定义令牌被 Authentication 模拟器以预期的 audience 和无签名令牌行为接受时，即表示完成。

由索引模型根据 Issue 内容生成。