FR: Support `webpush.fcm_options.link` for opening another domain.

### [REQUIRED] Describe your environment

* Operating System version: macOS 12.0.1
* Browser version: Chrome 96
* Firebase SDK version: 9.6.1
* Firebase Product: cloud messaging

### [REQUIRED] Describe the problem

According to the code below, when a user clicks the notification from FCM, it can only open the same domain.

https://github.com/firebase/firebase-js-sdk/blob/cdada6c68f9740d13dd6674bcb658e28e68253b6/packages/messaging/src/listeners/sw-listeners.ts#L130-L136

It would be fantastic if we could open a URL from another domain.

For the time being, I've devised a workaround: directly above the call to `firebase.messaging(),` insert your own `notificationclick`:

```js
self.addEventListener('notificationclick', event => {
const url = event.notification.data.FCM_MSG.notification.click_action

clients.openWindow(url)
})

firebase.messaging()
```

It is necessary to install an event listener sooner rather than later since it will be halted by the following line:

https://github.com/firebase/firebase-js-sdk/blob/cdada6c68f9740d13dd6674bcb658e28e68253b6/packages/messaging/src/listeners/sw-listeners.ts#L121-L122