[v2] Use external config instead of arguments in phpunit.xml

External config will be much more extendable and will virtually never break BC. The only optional argument will be the path to config (which will set by default to `config/allure.phpunit.config.php`).

Start by reading phpunit.xml and determine how its current arguments are passed into the integration. Trace the configuration entry point for the proposed config/allure.phpunit.config.php path; done means configuration is loaded externally while retaining only the optional config-path argument and preserving existing behavior.

Written by the indexing model from the issue text.