FR: Make `getRedirectResult(auth)._tokenResponse` public to get firstName and lastName with Apple signup

### [REQUIRED] Describe your environment

* Operating System version: any
* Browser version: any
* Firebase SDK version: 9.12.1
* Firebase Product: auth

### [REQUIRED] Describe the problem

When using Apple as a federated provider, there is no way to obtain `firstName` and `lastName`. We need both separated and cannot work just with `displayName`. While debugging we realised that with the following code we are able to obtain these attributes:

```javascript
const res = await getRedirectResult(auth)
const {firstName, lastName} = res._tokenResponse
```

But since `_tokenResponse` is clearly a private thing, we are a bit hesitant to use it (but will anyway for now. :-)).

Either make `_tokenResponse` a publicly available property or add `firstName` and `lastName` to the result of `getAdditionalUserInfo(redirect_result)`.