XSD Generator: Support emitting xs:any for fields not restricting the element name

[kwin]

Currently <xs:any> is only emitted for fields with type java.util.Properties or DOM (

modello/modello-plugins/modello-plugin-xsd/src/main/java/org/codehaus/modello/plugin/xsd/XsdGenerator.java

Lines 353 to 357 in 43aa6f4

	else if ( Properties.class.getName().equals( field.getType() )
	|| "DOM".equals( field.getType() ) )
	{
	writePropertiesElement( w );
	}

).

For the case of the Maven plugin descriptor the element configuration may contain children with arbitrary names (although in the Model it is incorrectly described as paramName, https://maven.apache.org/ref/3.8.6/maven-plugin-api/plugin.html#class_paramName).
The according MDO at https://github.com/apache/maven/blob/daa8e1690226fc385db3d448ad962997afe3dba8/maven-plugin-api/src/main/mdo/plugin.mdo#L340 just defines the placeholder paramName instead.

As XSD unfortunately doesn't support enforcing a complex type on elements with arbitrary names, the best one can come up with is xs:any.

I propose to support a value * for xml.tagName in https://codehaus-plexus.github.io/modello/modello.html#field which leads to allowing everything below in the XSD.

[kwin]

One other alternative is to emit XSD 1.1 which allows to put additional assertions on elements: https://stackoverflow.com/questions/14644747/is-it-possible-to-define-a-xsd-element-with-an-arbitrary-name-but-with-specific, but unfortunately XSD 1.1 parsers are not very common.

[kwin]

FTR: This is how Maven POM deals with configuration element: https://github.com/apache/maven/blob/18eff3baeb899ddd27b34635c8f0d7e47f5c027b/api/maven-api-model/src/main/mdo/maven.mdo#L2164 (it leverages type=DOM)