Refactor ECA events to "real" plugins
Problem/Motivation
Follow-up from #3278080: Action config validation may prevent models being saved copied from https://www.drupal.org/project/eca/issues/3278080#comment-14509390:
Having had another thought about this, there may also be an opportunity to provide some helpful feedback to modeller users. Let's say, we also turn ECA events into configurable plugins - ECA conditions already are - and we would run validation on all of them, we could e.g. tell a user that the field Entity needs to be empty or provide a string without token notation.
In other words, this may turn into a bigger cleanup where we could turn all 3 (events, conditions, actions) into configurable plugins and keep all the form stuff (build, validate, submit) to the plain Form API of Drupal and shift every BPMN specific modification to the border of the BPMN integration module. See also #3279525: Better handling of checkboxes in action config.
This would be another big one, but probably worth the effort, as the architecture would then be much cleaner and ready for further adoption by other integrations. But then we have to add special treatment of some weird validation and/or submit handlers that do stuff which is not suitable in the ECA context.