Implement Download handler using Storage class

# Description

Currently, data are stored as files (or using new storage classes - as database). But if storeexecuteresponse=true and reference is required from the client, we do reference to the file directly

in the future we plan to reference to OWS service

but maybe, we could refer to pywps download service, which would use pywps.inout.storage.Storage class

Start by reading the pywps.inout.storage.Storage class and the issue's proposed pywps download service. Determine how stored outputs and client references currently work when storeexecuteresponse=true. Done means the download handler can reference stored data through Storage rather than directly exposing files, with the intended future OWS integration clarified.

Written by the indexing model from the issue text.