Add PSR12 Validation with PHP CodeSniffer (PHPCS)

[szhajdu]

This issue proposes the addition of PSR12 validation using PHPCS to improve the code quality and maintainability of the project.

By integrating PHPCS into the CI process, we can catch basic issues before they are merged into the master and we can ensure that all new contributions adhere to the PSR12 standard, leading to a cleaner and more maintainable codebase.