[9.x] Standardize withCount() & withExists() eager loading aggregates.

[r-kujawa]

What does this PR do?

• Standardizes the way you can eager load your withCount() & withExists() aggregates.
• Adds asserts to existing tests to support this functionality☝🏼.

Why do we need this?

Before this PR

There are 3 different ways to pass parameters in with...() functions:

1. Passing relation strings (internally will check is_array($relations) ? $relations : func_get_args()).
2. Passing an array of relations (with or without a closure as the value).
3. Passing the first parameter as a relation string and second argument as a closure.

If we compare the with(), withCount() & withExists functions, this is what each of them currently support:

• with(): 1, 2 & 3.
• withCount(): 1 & 2.
• withExists(): 1 (with only 1 relation 😞) & 2.

The reason why I am writing this PR is because I ran into a situation where I supposed that since I was able to do it with one of these methods, why shouldn't I be able to do the same with any of the other 2 methods (that's where I got disappointed 😞).

I did notice that it would probably be best practice and most efficient to have way 1 & 3 removed and only support way 2. But that would result in some breaking changes (LMK if it sounds interesting and I will make a PR for 10.x).

With this PR

The 3 functions mentioned above will support all 3 different ways to pass the params.
🔴 The only caveat I might see is that the with() function includes a second parameter in it's signature (@param string|\Closure|null $callback) while the other 2 don't.

[nuernbergerA]

@taylorotwell This is actually a breaking change because the example from the docs no longer works now

use Illuminate\Database\Eloquent\Builder;
 
$posts = Post::withCount(['votes', 'comments' => function (Builder $query) {
    $query->where('content', 'like', 'code%');
}])->get();
 
echo $posts[0]->votes_count;
echo $posts[0]->comments_count;

will throw Undefined array key 1