Add Gitlab CI pipeline config.
This MR is prep work for migrating this repo to the Generated Data Platform org.
Once the repo is moved over, we'll be able to run CI using Gitlab runners and won't need to fallback to Github Action workflows.
How to register an account on GitLab. To prevent spam, new accounts are locked until approved by an admin or the approver bot. You can also file an unlock request to expedite access.
Support: mw:GitLab, how to host a project on GitLab, #wikimedia-gitlab on libera.chat, #GitLab on Phabricator.
This MR is prep work for migrating this repo to the Generated Data Platform org.
Once the repo is moved over, we'll be able to run CI using Gitlab runners and won't need to fallback to Github Action workflows.