referencedEntities should return referenced layout builder added entities

Problem/Motivation

When using something like default_content, the referenced entities for a node should include its inline blocks and other layout builder added content. This isn't the case. Because ContentEntityBase checks instanceof EntityReference. And layout builder added content is connected via a LayoutSectionItem.

Proposed resolution

Ideally we'd provide the referenced content in a seamless mechanism.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet