Plugin descriptor model should contain all elements understood by PluginDescriptorBuilder and correctly specify mandatory attributes

The plugin descriptor schema at https://maven.apache.org/ref/3.8.6/maven-plugin-api/plugin.html currently does not list all elements which are understood by PluginDescriptorBuilder

• v4Api (Boolean)

Also none of the elements are currently marked as required but in fact the PluginDescriptorBuilder unconditionally dereferences some elements. All those should be marked as required in the underlying model (also to generate a proper XSD).

In addition the configuration element need to be classified as type DOM to generate a proper XSD (compare with https://github.com/codehaus-plexus/modello/issues/264).