Add AllowedInclude::exists

[euoia]

It would have handy to have an AllowedInclude::exists (using withExists) which is in the same vein as AllowedInclude::count (which presumably uses withCount).

This can be implemented easily enough with AllowedInclude::custom but it seems like a good candidate for being included in the library.

If anyone else needs this functionality, here's how to do it with AllowedInclude::custom.

app/QueryBuilder/ExistsInclude.php:

<?php

namespace App\QueryBuilder;

use Illuminate\Database\Eloquent\Builder;
use Spatie\QueryBuilder\Includes\IncludeInterface;

class ExistsInclude implements IncludeInterface
{
    public function __invoke(Builder $query, string $relations)
    {
        $relationPieces = explode("Exists", $relations);
        if (count($relationPieces) !== 2) {
            throw new \Exception("Invalid 'Exists' relation name, expecting something like fooBarExists: {$relations}");
        }

        $relationName = $relationPieces[0];
        $query->withExists($relationName);
    }
}

Then use it like this:

        $this->allowedIncludes = [AllowedInclude::custom('shiftsTodayExists', new ExistsInclude())];

Assuming a relation like this:

    public function shiftsToday()
    {
        return $this->shifts()
            ->whereRaw("DATE(start_time) = DATE(NOW())")
            ->whereRaw("DATE(end_time) = DATE(NOW())");
    }