Add security-harness script

What does this MR do?

Following the same pattern we have in other projects, this MR proposes to add a scripts/security-harness script to avoid security fixes to be pushed to the public repository.

This script was copied from gitaly: https://gitlab.com/gitlab-org/gitaly/-/blob/master/scripts/security-harness

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines