Issue #3050804 by heddn, catch, drumm: Modified code checker

automatic_updates.module

automatic_updates.services.yml

config/install/automatic_updates.settings.yml

config/schema/automatic_updates.schema.yml

src/ReadinessChecker/ModifiedCode.php

src/ReadinessChecker/ModifiedFiles.php

src/ReadinessChecker/ReadOnlyFilesystem.php

src/Services/ModifiedFiles.php

src/Services/ModifiedFilesInterface.php

...est_automatic_updates/src/Controller/HashesController.php

...test_automatic_updates/test_automatic_updates.routing.yml

tests/src/Functional/AutomaticUpdatesTest.php

tests/src/Functional/ModifiedFilesTest.php

tests/src/Kernel/ReadinessChecker/ModifiedCodeTest.php

tests/src/Kernel/ReadinessChecker/ModifiedFilesTest.php