Easier way to inject ad in preprocess function or render array

Commit is looking good. Thanks for your contribution.
Couple of updates required

• Description needs to be added to README
• Add dependency injection for new class

Moved Nate's work into a git.drupalcode.org issue branch (with correct attribution) and added @VladimirAus's requested changes.

This is ready for review again.

Testing this, I ended up deep in the weeds of RenderCallback and TrustedRenderCallback interfaces before face-palming and remembering that all render functions are static, so @VladimirAus we can't use a DI pattern to inject entity_type.manager service into the class, but have to use \Drupal::service().

I pushed up latest changes, and this is tested.

Changes should go into 2.0.x please. Needs retargeting.