feat: add publish composer PHP

Presentation

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Closes #25

This MR adds a new php-publish job to the template, allowing the publication of Composer packages to the GitLab Package Registry.

It introduces the following variables:

• PHP_PUBLISH_ENABLED: Set to true to enable the job (default: false).
• PHP_PUBLISH_URL: The Composer repository URL (default: GitLab Package Registry).

Checklist

• General:
  • use rules instead of only/except
  • optimized cache configuration (wherever applicable)
• Publicly usable:
  • untagged runners
  • no proxy configuration but support http_proxy/https_proxy/no_proxy
  • no custom CA certificate(s) but supports $CUSTOM_CA_CERTS or $DEFAULT_CA_CERTS to declare custom CA certificate(s)
  • internet hostnames/urls only
• Used Docker images:
  • public images
  • official images (when possible)
  • latest tag (when possible)
• Documented:
  • README.md documents the new feature
  • kicker.json describes the new feature
• Tested & examplified:
  • (url to a project sample successfully using the new feature)