DEPL-6248: rules example jetty-plugin

Please don't use "WarSpec" - that sounds totally weird (like something that describes a WAR, but isn't actually a WAR...whereas this really _is_ the WAR file).

How about calling the deployable `jetty.War` and the deployed `jetty.DeployedWar`, which is also more in line with other plugins?

/cc @vpartington as I know he has strong opinions on the naming scheme ;-)