DATACMNS-456 - Introduced getAttribute(String) on RepositoryConfigurationSupport.

RepositoryConfigurationSupport now has a getAttribute(String) method to lookup String attributes on XML elements and annotations. The XML implementation converts the given attribute name in a lower-case, dashed representation for the lookup.

RepositoryConfigurationExtensionSupport now ha a generic postProcess(…) method to allow implementations that only need to work with String attributes unify their customizations into that single method.
DATACMNS-457 - MappingContext now provides information about managed types.

Added MappingContext.getManagedTypes() as well as ….hasPersistentEntity(Class) to access all types currently managed by the MappingContext and find out whether a type is managed by the context.
DATACMNS-458 - Added PersistentEntities value object.

Introduced simple value object to hide a set of MappingContexts and be able to access PersistentEntities from them.