Ability to "hide" entities

[sebersole]

For background, see https://hibernate.atlassian.net/browse/HHH-10992

Also there is a growing discussion on #3763

The general idea here is whether we want to allow certain entities to be hidden from queries and metamodel lookups. The stated use cases so far include:

1. envers provided entities
2. search would like to contribute entities for its own use as well.

#3763 already adds identifying the contributor of the mapping as well as the ability to specify different auto-DDL operations against different contributors.