[ECP-9489] Implement PHPStan and fix relevant issues

[candemiralp]

Description

This PR implements automated PHPStan analysis as a part of the workflows. As Magento 2 has its own limitations, we have decided to use bitexpert/phpstan-magento extension to provide platform specific requirements.

Besides implementing PHPStan to the workflows, major issues having Level 0 severity have been fixed. Mainly,

• Repositories have been used to load & save entities instead of the entity class' themselves.
• Dynamic property declarations have been refactored.
• Unit tests have been updated.

Tested scenarios

• Test workflows
• All payment flows including partial payments
• All modification flows including partial payments