Add generic support to indicate on which objects the Page Model API should use $ref logic

At this moment, the Page Model API version v1.0 contains this:

    private static final Class<?>[] KNOWN_PMA_ENTITIES = {
            ComponentWindowModel.class,
            HippoDocumentBean.class,
            HippoFolderBean.class,
            CommonMenu.class
    };

through which the PageModelSerializer 'knows' on which object types to include the $ref logic (flatten the structure).

However, it should be more flexible. Downstream projects should also be able to contribute their objects to be flattened via the $ref logic. Therefor, we can better replace the hardcoded KNOWN_PMA_ENTITIES logic with a marker interface

PageModelEntity

All objects implementing this marker interface will be flattened using the $ref logic