Allow validators passed to file_validate to use callable syntax

Problem/Motivation

Currently file_validate takes and array of validators keyed by function name. We can use the syntax accepted by \Drupal\Core\Security\DoTrustedCallbackTrait::doTrustedCallback to specify a class method or a service much like \Drupal\Core\Controller\ControllerResolver and \Drupal\Core\Render\Renderer::doCallback does.

Steps to reproduce

Proposed resolution

Add logic to look up file validators by class method or service definition and call doTrustedCallback() on them.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet