Add support for configuring Hibernate L2 Cache

As discussed with @snicoll on Gitter, it would be nice if Spring Boot could offer an easy way to use the JCache CacheManager created by Spring Boot with the Hibernate second-level cache.

Hibernate supports different L2 caching options which can be set by e.g. using specific properties like:

spring.jpa.hibernate.cache.region.factory_class=jcache
spring.jpa.hibernate.javax.cache.provider=org.ehcache.jsr107.EhcacheCachingProvider
spring.jpa.hibernate.javax.cache.uri=classpath:ehcache3.xml

However Hibernate also supports explicitly setting the cache manager which can be done via a custom HibernatePropertiesCustomizer like:

@Configuration
public class HibernateConfig {

    @Autowired
    private CacheManager cacheManager;

    @Bean
    public HibernatePropertiesCustomizer hibernatePropertiesCustomizer() {
        return hibernateProperties -> hibernateProperties.put(ConfigSettings.CACHE_MANAGER, cacheManager);
    }
}

It would be nice if Spring Boot could offer a specific property like spring.jpa.hibernate.use-explicit-cache-manager, which when set to true auto configures a HibernatePropertiesCustomizer which sets the cache manager automatically?

PS: This also relates to PR #14570 adding documentation how set up Hibernate second-level caching currently.

Start with the HibernatePropertiesCustomizer example and the listed spring.jpa.hibernate cache properties; review PR #14570 for the current documentation context. Done means an agreed configuration can make Spring Boot’s JCache CacheManager available to Hibernate’s L2 cache without the custom application bean, with the behavior documented and tested.