Current XML schema for plugin.xml can not be extended.
Indeed, internal configuration of components or extensions can not come with their schema and therefore do not benefit from validation or completion.

I propose to:

• change processContents of extension and component element from "skip" to "lax"
• search for files META-INF/runtime-schema in classpath for registering new schema (http://www.ametys.org/schema/*)
• use the namespace http://www.ametys.org/schema/plugin for elements in plugin.xml