Initially I defined just a org.hibernate.service.spi.ServiceContributor. But really we should allow contributions to each of the 3 Hibernate defined service registries:

• BootstrapServiceRegistryContributor - contribute to the BootstrapServiceRegistryBuilder
• StandardServiceRegistryContributor - contribute to the StandardServiceRegistryBuilder
• SessionFactoryServiceRegistryContributor - allow contributing to the building of a SessionFactoryServiceRegistry. Currently there is no builder for this, so adding this will take a little extra.