Facilitate a method to get all 'unique' containeritemcomponent's for some (sub)site

Through a REST call, the page composer should be able to get all available containeritemcomponent

Obviously, just returning all containeritemcomponent is not correct, as you might get many duplicates: A containeritemcomponent can be used in many containers.

Therefore, we need to expose a list of 'unique' containeritemcomponent

We define the uniqueness of the containeritemcomponent as defined by its 'hst:componentclassname' and 'hst:template'