Add support for nested hidden attributes in relations

[erezmus]

When serializing a model that has a relationship loaded, we might want to make a field in the related models hidden or visible. Currently we would need to do something like this:

$posts = Post::with('comments.author')->get();
$posts->each(fn($post) => $post->comments->each(fn($comment) => $comment->author->makeHidden('foo')));

The enhancement proposed would allow the use of the '.' and ',' notation, similar to the eager loading convention, in a shorter fashion:

$posts->makeHidden('comments.author:foo');

If there are multiple attributes, a comma is used:

$posts->makeHidden('comments.author:foo,bar');