Have a generic WPS encoding delegate interface

At the moment complex outputs are encoded in XML by using an EncoderDelegate.
By supporting raw outputs we move outside of the hyphotesis that the output is only XML or part of an XML document.
The same PPIO should be usable to encode, for example, an image in base64 (if someone is fool enough to ask for that) inside an xml document, but also as a raw PNG.

What I'm thinking is something like:

WPSEncoderDelegate extends org.geotools.xml.EncoderDelegate {
public void encodeRaw(OutputStream os);
}