Utilize Drupal\Component\Annotation\Plugin::parse() to merge in default values

As a part of #1966246: [meta] Introduce specific annotations for each plugin type, specific plugin types will now have their own annotation classes, and that is where their documentation will live.

It could also be a great way to kill off PluginManagerBase::$defaults (and therefore most usages of ProcessDecorator), as well as keeping the documentation and defaults all in one place.

I'm going to postpone this on #1967294: Add a dedicated @EntityType annotation, since that will be the best demonstration of it, and can be an easy second pass.