Validate @ConfigurationProperties on @Bean methods

Refactor `ConfigurationPropertiesBindingPostProcessor` to allow JSR-303
validation on `@ConfigurationProperties` defined at the `@Bean` method
level.

JSR-303 validation is now applied when a JSR-303 implementation is
available and `@Validated` is present on either the configuration
properties class itself or the `@Bean` method that creates it.

Standard Spring validation is also supported using a validator bean
named `configurationPropertiesValidator`, or by having the configuration
properties implement `Validator`.

The commit also consolidates tests into a single location.

Fixes gh-10803

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -1213,9 +1213,13 @@ to your fields, as shown in the following example:
 	}
 ----
 
-In order to validate the values of nested properties, you must annotate the associated
-field as `@Valid` to trigger its validation. The following example builds on the
-preceding `AcmeProperties` example:
+TIP: You can also trigger validation by annotating the `@Bean` method that creates the
+configuration properties with `@Validated`.
+
+Although nested properties will also be validated when bound, it's good practice to
+also annotate the associated field as `@Valid`. This ensure that validation is triggered
+even if no nested properties are found. The following example builds on the preceding
+`AcmeProperties` example:
 
 [source,java,indent=0]
 ----