Create an easy way to get layout sections for an entity.

I think this should be done using the context system, as in #2976148: Layout-based entity rendering should delegate to the correct section storage instead of hardcoding to either defaults or overrides. I will roll a patch based on that work.

The idea here is that, given N section storage plugins, you have to find one that will accept an entity and derive a list of sections from it. "Which plugin to use, given this entity" is a perfect use case for contexts. So to load sections from an entity, it would be something like this:

$contexts = [
  'entity' => EntityContext::fromEntity($entity),
];
$plugin_definitions = $section_storage_manager->getDefinitionsForContexts($contexts);
$plugin_id = key($plugin_definitions);
$plugin = $section_storage_manager->createInstance($plugin_id);
$plugin->setContexts($contexts);

$section_list = $plugin->getSectionListFromContext();

We could add a utility method to SectionStorageManagerInterface to streamline this (something like loadFromEntity()).

Postponing this, but here's a patch built on the other issue.

Now that #2976148: Layout-based entity rendering should delegate to the correct section storage instead of hardcoding to either defaults or overrides has been committed, this can be worked on.

Marking this as a duplicate