Expose backends/indexes through the ORM mapper APIs

Currently you can't access the backend/index from the ORM mapper, which is a bit annoying since it prevents from accessing the Elasticsearch REST client, in particular.

Ideally, we should probably introduce a public API representing the mapping (SearchMapping) accessible through either Search.mapping(entityManagerFactory) or Search.mapping(entityManager). Then we would declare in SearchMapping a getBackend(String backendName) method and a getIndexManager(String indexName) method.
The best candidate for implementing SearchMapping is probably HibernateOrmMapping.