Draft: Use external_url for gitlab-kas

What does this MR do?

Changes gitlab-kas to use a potentially separate external_url instead of the /-/kubernetes-agent sub-path from the gitlab-rails nginx config.

Related issues

#5784 (closed)

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

/cc @tkuah

Test Plan

1. Deploy omnibus-gitlab
2. Set gitlab_kas['enable'] = true in /etc/gitlab/gitlab.rb
3. Run gitlab-ctl reconfigure
4. Verify gitlab-kas is reachable via kas.GITLAB_EXTERNAL_URL
5. Set gitlab_kas['external_url'] = 'ANOTHER.DOMAIN'
6. Run gitlab-ctl reconfigure
7. Verify gitlab-kas is reachable via ANOTHER.DOMAIN

Equivalent MR/issue for the GitLab Chart

gitlab-org/charts/gitlab#2373 (closed)