Add Schema Php File Loader

[alexander-schranz]

As part of the Framework integration #113 and providing formats #115. We first implement the PhpFileLoader to load a Index Schema directly from a .php file:

<?php

use Schranz\Search\SEAL\Schema\Field;
use Schranz\Search\SEAL\Schema\Index;

return new Index('blog', [
    'id' => new Field\IdentifierField('id'),
    'title' => new Field\TextField('title'),
    'description' => new Field\TextField('description'),
]);