Create a new job @ integration.moodle.org to validate composer

Raised @ MDL-73269, though it shouldn't impact us much, because normally we have all changes to composer.json well controlled, we can create a new, easy check:

• Always run composer validate to ensure that there aren't errors, stuff out of sync... or other problems with the committed composer json & lock file.

Surely something to be deployed @ https://integration.moodle.org , for all supported branches, as a new post-commit job.

Job to perform it should be part of https://github.com/moodlehq/moodle-local_ci (there are a bunch of similar, conceptually, post-commit jobs there).