Enable cglib proxy for configuration classes if necessary

This commit updates code generation to customize the instantiation of
a configuration class that requires a proxy. Rather than instantiating
the raw class, the proxy is used.

Closes gh-29107

spring-test/src/test/java/org/springframework/test/context/jdbc/EmptyDatabaseConfig.java

@@ -31,7 +31,7 @@ import org.springframework.transaction.PlatformTransactionManager;
  * @author Sam Brannen
  * @since 4.1
  */
-@Configuration(proxyBeanMethods = false)
+@Configuration
 public class EmptyDatabaseConfig {
 
 	@Bean