Problem/Motivation
In #3332593: Adopt PluginExists validator in relevant places, we added many validation constraints to config schema ensuring that any plugin IDs mentioned in config do, in fact, exist.
One notable exception is Views' concept of display extenders. These things are, in a word, inscrutable. They are not used anywhere in core. Their existing config schema is extremely vague. What even are they? Do they need validation? They are clearly powered by plugins (\Drupal\views\Plugin\views\display_extender\DisplayExtenderPluginBase)...so, like, do they have plugin IDs stored in config anywhere...?
Proposed resolution
If display extenders are things that can have plugin IDs that are stored in config, add validation constraints for that, along with the relevant test coverage.
User interface changes
None.
API changes
None anticipated.
Data model changes
None anticipated.
Release notes snippet
TBD