added get collections list method to client

import it.inaf.ia2.vospace.ui.exception.VOSpaceException;

import it.inaf.oats.vospace.datamodel.NodeUtils;

import static it.inaf.oats.vospace.datamodel.NodeUtils.urlEncodePath;

import it.inaf.oats.vospace.datamodel.collections.NodeCollection;

import it.inaf.oats.vospace.datamodel.collections.NodeCollectionsWrapper;

import java.io.IOException;

import java.io.InputStream;

import java.io.StringWriter;

        return call(request, BodyHandlers.ofInputStream(), 200, res -> unmarshal(res, Node.class));

    }

    public List<NodeCollection> getNodeCollections(Optional<String> token) {

        HttpRequest request = getRequest("/collections", token)

                .header("Accept", useJson ? "application/json" : "text/xml")

                .GET()

                .build();

        return call(request, BodyHandlers.ofInputStream(), 200, 

                res -> unmarshal(res, NodeCollectionsWrapper.class)).getNodeCollections();        

    }

    public JobSummary startTransferJob(Transfer transfer, Optional<String> token) {

        HttpRequest request = getRequest("/transfers?PHASE=RUN", token)