Move the render callbacks used by RenderExampleController in a service

Problem/Motivation

Some of the render callbacks used by RenderExampleController use dependencies that are obtained from \Drupal. Those callbacks can be implemented as service methods, which at last could use the DI container.

Proposed resolution

Move the pre-render, post-render, and the lazy render callbacks used by RenderExampleController in service methods. Those methods can be marked as trusted callbacks with an attribute, which means that RenderExampleController::trustedCallbacks() can be removed too.