Doesn't need to be a validator since it's not actually invoked that way? See e.g. [ValidatePlaceholderEntries](https://github.com/xebialabs/community-plugins/blob/master/deployit-udm-plugins/utility-plugins/restrict-placeholders-to-dicts-plugin/src/main/java/ext/deployit/community/plugin/restrictplaceholders/planning/ValidatePlaceholderEntries.java) in the restrict-placeholders-to-dicts-plugin?