Proof of concept: passing objects instead of render arrays

Problem/Motivation

This is simply a proof of concept for #2602368-42: Allow objects that implement RenderableInterface to used interchangeably with render arrays. → . I want to work out the details in an MR here instead of adding noise to another issue.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet