Expose SpringValidatorAdapter contained in ValidatorAdapter via new SmartValidator method

[rstoyanchev]

ValidatorAdapter wraps SpringValidatorAdapter to expose it as a SmartValidator, and not as jakarta.validation.Validator to avoid competing with other jakarta.validation.Validator beans. This works fine since Sping MVC and WebFlux primarily need a Spring Validator. However, for the built-in method validation in 6.1, Spring MVC and WebFlux need the jakarta.validation.Validator and as a result the built-in method validation feature does not work currently with Boot autoconfig. See spring-projects/spring-framework#31082.

To address this, we are adding a new unwrap method on SmartValidator, and we'll need ValidatorAdapter to implement it to allow Spring MVC and WebFlux to unwrap ValidatorAdapter and access the validator instance it contains.

[wilkinsona]

Closing in favor of #37119.