This is part of the effort to get rid of the "runAsStage" step in the SDK pipeline, and use piperStageWrapper directly. The SDK pipeline currently needs to support for loading "old" extensions where the call() method had different parameters. The support for the exact API can and should be removed, however, having a mechanism for supporting old extension APIs seems beneficial in general.
Another crucial change is the deleteDir() call before unstashing at the beginning of the stage. Without this, the SDK pipeline fails to unstash, since apparently the workspace may not always be clean at that point.
* add wrapper for stages contained in library
`piperStageWrapper` provides a wrapper for stages which we may include into the library.
It will take care about extension capabilities, locking, node handling, ... which should be a capability of every stage contained in the library.
