CB: New hook lib for Postgres

One of the early steps that will eventually lead to a functional Config Backend for PostgreSQL is to create a hook library.

The goal of this ticket is to:

• create a new hook library itself
• create stub test framework
• create stub documentation for it
• update makefiles

The changes probably should be done in a way that does not include this hook in the release code. That would come later, once the lib is somewhat functional.