[2.1.0] Add a ContentBlock source for slots with a Content entity context

See also: https://www.drupal.org/project/media_library_block →

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

We'll borrow logic from the core EntityDisplayModeController which does indeed include a check for $entity_type->hasViewBuilderClass(). It's also got a check for $entity_type->get('field_ui_base_route') which we probably won't use because we're using of the Display Modes and not trying to create them.

t's also got a check for $entity_type->get('field_ui_base_route') which we probably won't use because we're using of the Display Modes and not trying to create them.

oh yes, we need to also select the display mode!