[7.x] Add 'morphWithCount' method

[gdebrauwer]

The morphWith method was added by PR #28647. It allows you to eager load relations on morphed models.

This PR adds a morphWithCount method that allows you to eager load relationship counts on morphed models. It uses the same syntax as the morphWith method.

An example:

Comment::query()
    ->with(['commentable' => function (MorphTo $morphTo) {
        $morphTo->morphWithCount([Post::class => ['likes']]);
    }])
    ->get();

[taylorotwell]

Would you mind sending in a documentation PR for this? Thanks.