enable systemd unit to handle async tasks (symfony workers)

similar to adullact/puppet-tajine#39

resources:

• https://symfony.com/doc/current/messenger.html#consuming-messages-running-the-worker
• https://symfony.com/doc/current/messenger.html#deploying-to-production
• https://symfony.com/doc/current/messenger.html#prioritized-transports
• https://symfony.com/doc/current/messenger.html#systemd-configuration
• https://jolicode.com/blog/symfony-messenger-systemd
• https://afup.org/talks/4039-des-workers-php-avec-systemd-et-symfony-messenger
  • https://github.com/pyrech/worker-php-conf/tree/main
  • https://speakerdeck.com/pyrech/des-workers-php-avec-symfony-messenger-et-systemd