add a rocket league "time played" counter

Use the steam API:

```
GET http://api.steampowered.com/IPlayerService/GetOwnedGames/v0001/?key=<mykey>&format=json&input_json={"steamid": 76561198035661237, "appids_filter": [252950]}
```

Use a Vercel function to request the steam API with a cache of ~12h to reduce potential rate-limiting.