ci(SAST): add GitLab template

as seen on

• https://lab.allmende.io/ecobytes/web/-/merge_requests/new/diffs?merge_request%5Bdescription%5D=Configure+SAST+in+%60.gitlab-ci.yml%60+using+the+GitLab+managed+template.+You+can+%5Badd+variable+overrides%5D%28https%3A%2F%2Fdocs.gitlab.com%2Fee%2Fuser%2Fapplication_security%2Fsast%2F%23customizing-the-sast-settings%29+to+customize+SAST+settings.&merge_request%5Bsource_branch%5D=set-sast-config-1

after clicking the link to automatically enable SAST.

While the output of the GitLab formatter for .gitlab-ci.yml is certainly interesting, I don't trust it at this scale right now for a configuration that is known to work. What originally motivated this adaption of the comparatively small overall addition to the CI manifest.