SpringBoot Allow PersistenceUnitPostProcessor to be configured on EntityManagerFactoryBuilder

Enable persistence unit post processors configurable in EntityManagerFactoryBuilder

This PR enables persistence unit post processors configurable by implementing EntityManagerFactoryBuilderCustomizer.java. It works only for org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean#internalPersistenceUnitManager.

Comment From: pivotal-issuemaster

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Comment From: pivotal-issuemaster

@JohnNiang Thank you for signing the Contributor License Agreement!

Comment From: JohnNiang

@snicoll

If the changes are applied, so that we could customize MutablePersistenceUnitInfo as follow:

@Bean
EntityManagerFactoryBuilderCustomizer entityManagerFactoryBuilderCustomizer() {
    return builder -> {
        builder.setPersistenceUnitPostProcessors(pui -> {
            pui.addManagedClassName("customized.attribute.converter.class.name");
                        // other operations on MutablePersistenceUnitInfo
                        pui...
        });
    };
}

A scene is that we can register attribute converters into org.hibernate.jpa.boot.spi.EntityManagerFactoryBuilder programmatically. Because default fetching managed class names perform is scanning from class path only.

Comment From: snicoll

@JohnNiang thank you for making your first contribution to Spring Boot.