Remove CKEditor5PluginContextualInterface and CKEditor5PluginContextualValidationInterface in favor of annotation

Problem/Motivation

Removing CKEditor5PluginContextualInterface

Quoting #3215506-11: Plugins should be enableable based on toolbar configuration

We can't just conditionally enable ckeditor5.image though, because if there's no PHP code, one cannot implement CKEditor5PluginContextualInterface either. So I see at least 4 solutions:

1. The simplest possible approach here AFAICT is to move the elements annotation from ckeditor5.image to ckeditor5.imageUpload.
2. Merge ckeditor5.image and ckeditor5.imageUpload into a single plugin definition again. This is also still very simple. It would just mean one fewer CKEditor 5 plugin definition defined in YAML.
3. Add class to ckeditor5.image and add a tiny amount of code to implement CKEditor5PluginContextualInterface.
4. Add infrastructure to conditionally enable a particular CKEditor plugin from YAML only. Something like enableIfToolbarItemIsPresent: foobar. This is an API addition though. It feels a bit logic-y. Which is bad for an annotation.

I advocated against

4. Add infrastructure to conditionally enable a particular CKEditor plugin from YAML only. Something like enableIfToolbarItemIsPresent: foobar. This is an API addition though. It feels a bit logic-y. Which is bad for an annotation.

… but I actually do think this could be a good idea. Because it will actually allow writing less PHP code:

ckeditor5.media

would get

conditions:
  filter: media_embed

ckeditor5.mediaLibrary

would get

conditions:
  filter: media_embed
  toolbarItem: drupalMedia

ckeditor5.image

would get

conditions:
  toolbarItem: uploadImage
  imageUploadStatus: true

ckeditor5.imageUpload

would get

conditions:
  toolbarItem: uploadImage
  imageUploadStatus: true

Consequence

All CKEditor5PluginContextualInterface implementations would disappear!

Removing CKEditor5PluginContextualValidationInterface

Better still, this would allow us to remove CKEditor5PluginContextualValidationInterface as well! The only use for that so far is \Drupal\ckeditor5\Plugin\CKEditor5Plugin\MediaLibrary::validateFilterForm(), which explicitly checks whether the drupalMedia filter is enabled. That requirement/validation logic can easily be inferred from that metadata.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes