include MetacatHandlerPlugin interface so that other projects [semtools] can extend the actions handled by Metcat without editing the codebase directly. The SemtoolsPlugin is the prototype for this and can be registered using the Metacat admin interface. Note that the plugin implementation class and supporting jars must be included in the webapp in order for the request to be handled correctly (the semtools build is injecting these as part of it's build process).