Configure external GitLab URL for KAS

Allow external GitLab URL configuration for KAS

Add configuration key to set `gitlab.external_url` in KAS in support of
cross origin resource sharing (CORS) requests.

Changelog: changed

What does this MR do?

In Add cookie-based user access to the k8s API proxy (gitlab-org/cluster-integration/gitlab-agent!841 - merged) we've introduced a new gitlab.external_url configuration setting which should contain the URL of the GitLab frontend. This is required to make CORS requests work. If KAS is hosted on the same domain as GitLab this setting is not required, but also doesn't hurt.

I've made a similar change in the charts: kas: add external url to KAS service config (gitlab-org/charts/gitlab!3015 - merged)

Related issues

• Update KAS Chart with new `external_url` setting (gitlab#395540 - closed)
• Frontend access to Kubernetes API through KAS (&8038)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened