[8.x] Added orderPivotBy for BelongsToMany relation

[smujaddid]

This PR will allow us to directly order the query results of BelongsToMany relation such as:

class Tag extends Model
{
    public $table = 'tags';
}

class Post extends Model
{
    public $table = 'posts';

    public function tags()
    {
        return $this->belongsToMany(Tag::class, 'posts_tags', 'post_id', 'tag_id')
            ->using(PostTagPivot::class)
            ->withTimestamps()
            ->withPivot('flag');
    }
}

class PostTagPivot extends Pivot
{
    protected $table = 'posts_tags';
}

// Somewhere in a controller
public function getPostTags($id)
{
    return Post::findOrFail($id)->tags()->orderPivotBy('flag', 'desc')->get();
}

Using orderPivotBy will allow us to not having to append the Tag class table name explicitly in case the flag attribute exists in both Tag and PostTagPivot. The results will be ordered based on flag value.

Note: I will also make a PR for [7.x] branch