GEOS-6530 Implemented PagedUnique WPS process

Implementation of a Paginating Unique WPS Process, that extracts unique values from a SimpleFeatureCollection attribute, with optional pagination.
The output is a json with the list of unique values, the total size of the list (without taking pagination in account)
Could you move this into the PPIO? There must be a separation of concern, the PPIO does the encoding, the object is just the model. E.g., it must be possible to add other encodings, for example XML based on XStreamPPIO, without having to touch the model.