Include plugin validation of ECA

Problem/Motivation

Once #3315518: Allow usage of tokens in number fields got fixed, the Core modeller needs to include ECA's own validation mechanic on its plugin validation. Currently, the Core modeller directly calls for plugin form validation in Drupal\eca_cm\Form\EcaPluginForm::validateForm().

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes