Create a validator to add a warning if updated extensions have database updates

Problem/Motivation

For Project Browser #3300061-10: [policy, no patch] Determine Composer validation rules for installing through Composer
it was determined that the user should be made aware that there are database updates in updated packages and consent is required to proceed further.

Proposed resolution

1. Implement a listener to the StatusCheckEvent to add a warning if there are db updates in the updated packages.
2. Create a kernel test to prove the warning is added. see \Drupal\Tests\package_manager\Kernel\PackageManagerKernelTestBase::assertStatusCheckResults

To make the listener we can start by:

1. copying \Drupal\automatic_updates\Validator\StagedDatabaseUpdateValidator and moving it into package manager.
2. in the copy change `getSubscribedEvents` to only subscribe to `StatusCheckEvent
3. in the copy remove the if (!$stage instanceof CronUpdater) { this check will apply to all stages.
4. instead of addError use addWarning because this should not indicate that the update should proceed just that the user should be warned.
5. This is the first StatusCheckEvent subscriber that should only run its check if the update is already staged.

   I think how to check that would be use \Drupal\package_manager\Stage::getStageDirectory

   so maybe something like

   if (!file_exists($event->getStage()->getStageDirectory())) {
     // No staged updates exist, therefore we don't need to run this check.
     return;
   }
   

for the test you can also start by copying \Drupal\Tests\automatic_updates\Kernel\ReadinessValidation\StagedDatabaseUpdateValidatorTest

Remaining tasks

1. Determine if \Drupal\automatic_updates\Validator\StagedDatabaseUpdateValidator should now extend this new class. We will probably do this in a follow-up
2. In a follow-up remove the database update specific checking in \Drupal\automatic_updates\Form\UpdateReady::buildForm and just fire the StatusCheckEvent and display the warnings