Add a ContentEntity source for slots with a Content entity context

Problem/Motivation

Today, we can only add block plugins to slots.

When using UI Patterns 2 in layout builder override or in Ckeditor5 in a content field (when it will be available), it may be useful to also be able to add content block (content entities).

Proposed resolution

Add a new source for slots

With content entities (not only content blocks).

So, we may need to pick the content entity type first, among the renderable ones:

• Node
• Taxonomy Term
• User
• Block Content
• Comment
• Media
• Workspace
• Message

So, not those ones: Path Alias, File, Menu Link.

Let's not set a manual "allowed list", we need a dynamic way, maybe by checking if view_builder attributes exist in the content entity type (this is excluding User but that may be OK):

$ grep -r "view_builder' =>" core/modules/*/src/Entity
core/modules/block_content/src/Entity/BlockContent.php:    'view_builder' => BlockContentViewBuilder::class,
core/modules/comment/src/Entity/Comment.php:    'view_builder' => CommentViewBuilder::class,
core/modules/contact/src/Entity/Message.php:    'view_builder' => MessageViewBuilder::class,
core/modules/media/src/Entity/Media.php:    'view_builder' => EntityViewBuilder::class,
core/modules/node/src/Entity/Node.php:    'view_builder' => NodeViewBuilder::class,
core/modules/taxonomy/src/Entity/Term.php:    'view_builder' => EntityViewBuilder::class,
core/modules/workspaces/src/Entity/Workspace.php:    'view_builder' => WorkspaceViewBuilder::class

Once the content entity type is picked, what do we do?

• for media, if media_library module is available, we open a media library form element?
• for others:
  • we select the bundle, and then we have an autocomplete reference field?
  • directly the autocomplete reference field?
  • something fancier?

Do we need to be careful with recursivity, like the Entity Reference Field is doing (break after X loops)?

Add a new context

To make this source only available when we are in a content entity, in order to avoid dependency from config to conrent.

This context must be injected in Layout Builder Override and UI Patterns Field (and the upcoming UI Patterns Ckeditor5).