Config validation for ECA models

Rebased for 3.0.x

We're pretty close now. There are a few missing schema errors left when PhpUnit is trying to go through all our test cases. But I have merged all other related issues from 🌱 [META] Implement complete coverage of validatable config schema across plugins and entities Active so that we can finally go through all of these remaining issues.

@michaellander thank you so much for everything you've done here together with your team. This has been a massive effort.

This is now mostly done, only 4 tests are still failing like with this message:

1) Drupal\Tests\eca_workflow\Kernel\WorkflowTransitionTest::testExecuteWithNoEntity
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "eca_workflow_transition:editorial" plugin does not exist. Valid plugin IDs for Drupal\Core\Action\ActionManager are: node_make_unsticky_action, node_make_sticky_action, node_promote_action, node_unpromote_action, user_unblock_user_action, user_remove_role_action, user_cancel_user_action, user_block_user_action, user_add_role_action, entity:unpublish_action:node, entity:save_action:node, entity:save_action:user, entity:publish_action:node, action_message_action, action_goto_action, action_send_email_action, entity:delete_action:node
/builds/project/eca/web/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php:53
/builds/project/eca/web/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryCachedTrait.php:28
/builds/project/eca/web/core/lib/Drupal/Core/Plugin/Factory/ContainerFactory.php:16
/builds/project/eca/web/core/lib/Drupal/Component/Plugin/PluginManagerBase.php:85
/builds/project/eca/src/PluginManager/Action.php:178
/builds/project/eca/modules/workflow/tests/src/Kernel/WorkflowTransitionTest.php:101

I'löl fix that over the weekend, and then we should be able to finish this off. What a big one !!!

Yeah, all tests are green. What an achievement. Thank you so much @michaellander, @lammensj, and everybody else who made this possible.