Apple token validation fails because no userinfo_endpoint

Currently if you try to validate an apple token, building the document fails on

```
        userinfo_endpoint: Map.fetch!(document_json, "userinfo_endpoint"),
```

Because Apple does not provider this property on their config (https://appleid.apple.com/.well-known/openid-configuration). 

Ideally this should just be optional, or configurable, so it could work with Apple.