[INTERNAL] moved XStreamExtensionProvider to Saros Core

Change-Id: Ifcf5a9965e4f9bf3cabfbb02458f967e77955250
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1436
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>