Key transformer

It would be handy to have an ability to specify the key transformer:
```php
SharedData::keyTransformer(function ($key) {
    return Str::camel($key);
});
```

So that:
```php
share(['auth_user' => auth()->user()]);
```
Will be transformed into:
```json
{
    "authUser": {
        "id": 1,
        "firstName": "Ilya",
        "lastName": "Sakovich"
    }
}
```