feat: add extract-script job template

templates/extract.yml

+# this job extracts the bash script from the given template(s)
+extract-script:
+  image: alpine
+  stage: .pre
+  script:
+    - echo "#!/bin/bash" > script.sh
+    - sed -n '/BEGSCRIPT/,/ENDSCRIPT/p' "$GITLAB_CI_FILES" | sed 's/^  //' >> script.sh
+    - export LC_ALL=C.UTF-8
+  artifacts:
+    when: always
+    name: extracted template script
+    expire_in: 1h
+    paths:
+      - script.sh
+  rules:
+    # exclude merge requests
+    - if: $CI_MERGE_REQUEST_ID
+      when: never
+    # enabled if $GITLAB_CI_FILES is set
+    - if: $GITLAB_CI_FILES