Add the Symfony preload.php file

[xvilo]

PHP Preloading was introduced in PHP-FPM with PHP 7.4, Symfony started supporting this in September 2020. The config/preload.php file is a generic auto generated file by the Symfony FrameworkBundle, this file should be committed in the repository.

We would like to start doing tests with Repman and PHP Preloading to see if it benefits and how much.

[codecov[bot]]

Codecov Report

Merging #610 (13480d9) into master (8ec5c90) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 13480d9 differs from pull request most recent head 1a0298d. Consider uploading reports for the commit 1a0298d to get more accurate results

@@            Coverage Diff            @@
##             master     #610   +/-   ##
=========================================
  Coverage     99.16%   99.16%           
  Complexity     1910     1910           
=========================================
  Files           301      301           
  Lines          6072     6072           
=========================================
  Hits           6021     6021           
  Misses           51       51           

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