kas: add external url to KAS service config

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.

Related issues

• Update KAS Chart with new `external_url` setting (gitlab-org/gitlab#395540 - closed)
• Frontend access to Kubernetes API through KAS (gitlab-org&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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

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 omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.