Allow adding a body class in a route / controller

Problem/Motivation

Adding a body class in a controller to indicate for example, which type of entity or page is currently handled doesn't seem to be possible
Using hook_preprocess_html() isn't helpful as it doesn't have a good way to determine the current controller / entity.

See https://drupal.stackexchange.com/questions/210376/how-can-i-add-a-css-cl... for similar questions

So the problem in behind is to get the information from the controller into the page / html.

Maybe this should be extended to a wider context of being able to change / attach page metadata in the controller, probably through the render array?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet