allow plugins to define @Configuration beans

adding a second `ConfigurationClassPostProcessor` following these comments: https://github.com/grails/grails-core/commit/61219075e8272c0c1ed9f4a89e061295dd72586d#commitcomment-8098202

this change allows plugins to define `@Configuration` beans, meaning they are getting post-processed properly.

when the `ConfigurationClassPostProcessor` that gets automatically registered by spring is applied, bean definitions from plugins are not yet available.