I.e. expose an (immutable) set of SearchIndexedEntity, from which we can get access to the index manager and index descriptor.
We need this for the Search 5 migration helper to handle its weird way of overriding analyzers in particular.
While we're at it, let's introduce similar metadata to the JavaBean mapper, because we'll need that in the migration helper tests, too.
I think that could be useful for Infinispan too…