Add option for version watch script

Please Note: This PR has low priority.

gitlab variable VERSION_WATCH to specify a script (located in the respective gardenlinux package repo), that prints the latest upstream version. If VERSION_WATCH is specified, then the build will abort if a newer upstream is found than the latest available tarball in pristine-ifs.

Testing with frr: gardenlinux-package-frr!4

Changes only concerns package builds that use existing ORIG_TAR flow.