Avoid unnecessary CGLIB processing on configuration classes

[jhoeller]

If a @Configuration class does not declare (or inherit) any instance-level @Bean methods, generating an enhanced subclass with CGLIB is not adding any value. This is particularly likely for "holder"-like configuration classes with just imports, not least of it all common @SpringBootApplication classes. We can entirely skip CGLIB processing for such classes, also not incurring unnecessary potential for CGLIB class cache failures/fallbacks (#34423).