XML format option for view parameters

This is an idea to enhance GeoServer view parameters to allow an XML format with more complex values definitions without the limit on not using reserved characters as the current plain format.

Current GeoServer view params documentation:

Motivation:
To have the ability of sending complex values into view parameters, like comma separated sublists, semicolon and comma separated inner sub-parameters, CSV sets, JSON/XML, etc.

Proposed XML alternative format example:

viewParamsFormat new optional parameter definition:

• Selects the view parameters format implementation to use by its identifier.

• Optional parameter, if not set the default character separated format will be used supporting backward compatibility.

XML tags/attributes definition:

• VP: the root xml element tag for View Params. This was added to comply with the XML format validation.

• PS: the parameters set XML element for the positional layer. If there are no parameters for the current layer this must be set as empty element <PS/>

• P: the parameter definition XML element, including the parameter name as the n attributer and the value as the XML element child value text.

• n: the parameter name attribute inside the 'P' parameter definition element.

Note: If a layer doesn't have parameters to set we just set an empty PS element as seen on the example: <PS/>