Expose NodeTypeRegistry over the InternalHippoRepository interface so (new) AutoExport module can register a listener on it

The new AutoExport module will be using JCR persisted Journal Events instead of 'live' JCR events.

This change however 'loses' the ability to track/monitor NodeType changes as those are not recorded as persisted Journal events.

To fix this, a NodeTypeRegistryListener can be added to the NodeTypeRegistry, which then can itself create a persisted Journal Event (with some trickery, using the JCR Event#userData payload).

To be able to access the NodeTypeRegistry however, the InternalHippoRegistry interface needs to expose it (and RepositoryImpl make its method public instead of protected).