KEYCLOAK-18747 Turn MapClientEntity into interface + introduce delegates

Given that the Map*Entity is turned into an interface, it makes more sense
to use non-primitive types to allow for null values. This enables signalizing
that an entity does not define a particular value, and builds a base for definition
of instances with defaults: If a value is not present in the queried instance
(i.e. is `null`), the value would be obtained from a delegate containing
the defaults.