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