Add Spring Boot integration

Create convenient infrastructure for using **validator4j** in Spring Boot. Mainly we need a certain validator implementation and ControllerAdvise to respond to requests with validation errors.

Start by reviewing validator4j's existing validation API and the repository's generated validation flow, then compare them with Spring Boot request validation and ControllerAdvice conventions. The issue is complete when a defined Spring Boot integration provides the required validator implementation and returns validation errors from requests, with documented behavior and tests.

Written by the indexing model from the issue text.