Use lazy builders for homebox portlet contents

Problem/Motivation

https://www.hashbangcode.com/article/drupal-9-using-lazy-builders to separate the homebox from its contents

This might already be possible for individual portlet types by implementing that in
HomeboxPortletTypeXXX::buildDisplayContent()

Steps to reproduce

Proposed resolution

- Provide an example implementation for a Portlet Type using this functionality

Remaining tasks

User interface changes

API changes

Data model changes