[FR] Expose `link` / `link_android` in notifications for deep linking

### Problem statement
Deep linking on Android requires the `data` field in the `Intent` to be set. As described in [this SO post](https://stackoverflow.com/questions/63378233/how-to-send-firebase-notifications-with-uris-to-implement-deep-linking) or firebase/firebase-android-sdk#6703, the only way to populate the the field in notifications created automatically by the Firebase Android SDK (when the app is in the background) is by setting the `gcm.n.link` or `gcm.n.link_android` keys in `data`, which have reserved prefixes and are therefore not _supposed_ to be used (although setting them works, at least today).

_Originally posted by @lehcar09 in [#6703](https://github.com/firebase/firebase-android-sdk/issues/6703#issuecomment-2666361645)_:
> Alternatively, we can file a feature request to support the `link` or `link_android` field. I’ll get back to you once I hear back from them.

So, this is the FR to officially support these :)

### Strawman

A strawman proposal would be to add `link` properties to the `Notification` / `AndroidNotification` classes, which would then be mapped to `gcm.n.link` / `gcm.n.link_android` in the JSON payload. Equivalently, in the HTTP API instead of this:

```JSON
{
"message": {
"notification": {
"title": "Lorem",
"body": "Ipsum"
},
"data": {
"gcm.n.link": "app://deep/link"
},
}
}
```
It would be this:
```JSON
{
"message": {
"notification": {
"title": "Lorem",
"body": "Ipsum",
"link": "app://deep/link"
}
}
}
```
or this:
```JSON
{
"message": {
"android": {
"notification": {
"title": "Lorem",
"body": "Ipsum",
"link": "app://deep/link"
}
}
}
}
```

### Alternatives
The workaround suggested [here](https://github.com/firebase/firebase-android-sdk/issues/6703#issuecomment-2683556930) of using `setAction` doesn't work, because actions needs to be declared in the Android manifest in order to match the intent, which conflicts with dynamically registering deep link targets.

Thanks!

Start by locating the Notification and AndroidNotification classes and the HTTP API payload mapping. Review how existing notification properties are serialized, then determine how link and link_android should map to gcm.n.link and gcm.n.link_android. Done means the officially supported fields produce the required deep-link notification payloads.

由索引模型根据 Issue 内容生成。