Add viewParams option

[thinkingmik]

Hi, I need to pass viewParams variable to geoserver GetFeature call request, because I would use these parameters in a sql-view layer.

The request payload will be something like:

<wfs:GetFeature xmlns:wfs="http://www.opengis.net/wfs" 
      service="WFS" version="1.1.0"
      viewParams="from:1613057992957;to:1613057992957"
      outputFormat="application/json">
	<wfs:Query typeName="myworkspace:test_layer_view" srsName="EPSG:4326">
		<ogc:Filter xmlns:ogc="http://www.opengis.net/ogc">
			<ogc:BBOX>
				<ogc:PropertyName>geometry</ogc:PropertyName>
				<gml:Envelope xmlns:gml="http://www.opengis.net/gml" srsName="EPSG:4326">
					<gml:lowerCorner>10.897064208984375 45.29469404344385</gml:lowerCorner>
					<gml:upperCorner>12.215423583984375 45.63948755947343</gml:upperCorner>
				</gml:Envelope>
			</ogc:BBOX>
		</ogc:Filter>
	</wfs:Query>
</wfs:GetFeature>

I think it could be a very useful enanchment.

Thank you!