Add ability to specify artifacts to search for supplementalModels

Currently, supplementalModels need to be loaded either from the current project or from a direct dependency of the maven-remote-resources-plugin. Due to issue with maven plugin classloaded, if one project in a reactor requires different models loaded from different jars, this doesn't work. It would be good to add:

<supplementalModelArtifacts>
<supplementalModelArtifact>groupId:artifactId:version</supplementalModelArtifact>
</supplementalModelArtifacts>

type thing to load them separate just from config.

(btw: this is ONE of the blockers from CXF not being able to use the apache parent pom. PMD, eclipse, and Checkstyle plugins suffer from the same problem. Will fix those some other time.)