[php] Add PSR-12 rule set

Created by: ybelenko

I've added PSR-12 rule set to Php-CS-Fixer config since it's officially supported now. The config file has been renamed to .php-cs-fixer.dist.php to be able to overwrite it with local config file without dist part.

I don't know why we have so many rules since we agreed to follow PSR-12 only at the first place. New config:

/**
 * @generated
 * @link https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/HEAD/doc/config.rst
 */
$finder = PhpCsFixer\Finder::create()
    ->in(__DIR__)
    ->exclude('vendor')
    ->exclude('test')
    ->exclude('tests')
;

$config = new PhpCsFixer\Config();
return $config->setRules([
        '@PSR12' => true,
        'phpdoc_order' => true,
        'array_syntax' => [ 'syntax' => 'short' ],
        'strict_comparison' => true,
        'strict_param' => true,
        'no_trailing_whitespace' => false,
        'no_trailing_whitespace_in_comment' => false,
        'braces' => false,
        'single_blank_line_at_eof' => false,
        'blank_line_after_namespace' => false,
        'no_leading_import_slash' => false,
    ])
    ->setFinder($finder)
;

If somebody knows why we have disabled:

        'braces' => false,
        'single_blank_line_at_eof' => false,
        'blank_line_after_namespace' => false,

Also added rule no_leading_import_slash => false because there are model classes with leadings slashes in imports. No clue why, but still.

Fixer run output:

vendor/bin/php-cs-fixer fix --allow-risky=yes -vv
PHP CS Fixer 3.8.0 BerSzcz against war! by Fabien Potencier and Dariusz Ruminski.
PHP runtime: 8.0.16
Loaded config default from "/Users/ybelenko/Sites/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/.php-cs-fixer.dist.php".
............................................................                                                                                    60 / 60 (100%)
Legend: ?-unknown, I-invalid file syntax (file ignored), S-skipped (cached or empty file), .-no changes, F-fixed, E-error

Fixed all files in 2.246 seconds, 28.000 MB memory used

I would remove the old .php_cs file from samples right in current PR, but the CI workflow will be failed instantly...

Related #4726

cc @jebentier @dkarlovi @mandrean @jfastnacht @renepardon

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