Add support of an optional printProviderConfig parameter

Sometimes it can be useful to send additional parameters to the print proxy. This PR suggests to add an optional ``printProviderConfig`` in CGXP's Print plugin config as in the following example:
```
    {   
        ptype: "cgxp_print",
        legendPanelId: "legendPanel",
        featureProvider: "featureGridBL",
        outputTarget: "left-panel",
        printURL: "${request.route_url('printproxy', path='')}",
        mapserverURL: "${mapserverProxyUrl}", 
        checkLegend: false,
        % if 'anonymous' in request.params:
        printProviderConfig: {
            anonymous: true
        },  
        % endif
        options: {
            labelAlign: 'top',
            defaults: {
                anchor:'100%'
            },  
            autoFit: true
        }   
    }
```