Add parameters for groupId and artifactId to make it possible to copy only one artifact from a staging repository

If we take the current version parameter and add groupId and artifactId parameters, we can create a path to the artifact within the source repository. This enables us to copy only a part of the staging repository.

If groupId is omitted, the whole staging repo is copied, just like now.

If the groupId is supplied but the artifactId is omitted, all artifacts for that group is copied.

If groupId and artifactId is supplied, only that artifact is copied. In fact only that version of that artifact is copied, since we already have the mandatory version parameter.