Draft: feat: validate GitLab public server url config

What does this MR do?

This MR add validation for the -gitlab-server config option (which is currently required)

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