Allow external URL in Custom CI config path
Description
In many cases it is desirable to keep the build config centrally managed and not included as part of the repo. For example to reduce chance of errors because developers accidentally change/remove .gitlab-ci.yml, or in order to quickly roll out changes to the build config for multiple projects. Currently this is hard to manage as the .gitlab-ci.yml needs to be within the code repo of each project, and we have to build tools around it which creates/updates the file.
Being able to load the .gitlab-ci.yml from an external URL would also allow us to generate dynamic build configs based on parameters outside of Gitlab.
Proposal
My proposal is to simply allow an external URL to be entered in the "Custom CI config path" textbox which already exists in the project settings. If this is set, any .gitlab-ci.yml inside the repo should be ignored. Ideally the CI/CD variables should be available so we can use an URL like this: https://some-server/build-configs/CI_PROJECT_ID/CI_COMMIT_REF_NAME.