Extend the XWikiPluginInterface with beginParsing and endParsing

The plugins should be able to receive some kind of hooks/notifications when a response is being generated, by implementing beginParsing and endParsing, called before and after the template responsible for generating the response is parsed, in com.xpn.xwiki.web.Utils#parseTemplate.

beginParsing could be used to perform request-specific initialization, and endParsing could be used to cleanup request-specific objects or to alter the resulting string.

Although Utils#parseTemplate is not the best place to call these methods, the current structure of the rendering process does not allow a better place.