Add support for validation of XML submission files
This is a follow up to the submission XML schemas along with the validation artisan command (see https://github.com/Kitware/CDash/pull/2335) that were recently introduced. The changes in this PR add the option to use this validation process on incoming submission files, although this option is disabled by default. A test has been added to verify these changes, and the docs have been updated to reflect the new configuration option.
Moving this to the 3.7 release since it isn't quite finished.
@josephsnyder I just pushed an update which makes the code cleaner and hopefully gets rid of some of the complexity that might've been tripping you up. I also resolved all of the PHPStan errors. I didn't take a detailed look into why the submission validation test is failing, but I can dig into that a bit more if you'd like.