Add internal and external URL config for KAS

What does this MR do?

Adds auto-configuration and manual overrides for the following gitlab.yml fields required for GitLab-backend's awareness of KAS:

• gitlab_kas.enabled -- is there a KAS?
• gitlab_kas.external_url -- the URL for external clients (agentk)
• gitlab_kas.internal_url -- the URL for internal clients (GitLab rails)

Related issues

gitlab-org/gitlab#299850 (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 with KAS_ENABLED=1: https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/266128551

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: gitlab-org/omnibus-gitlab!5016 (merged)

Test plan

1. Set up kind as per https://docs.gitlab.com/charts/development/kind/#nginx-ingress-nodeport-without-ssl
2. Installed GitLab chart with KAS:

   helm upgrade --install gitlab . \
     --set global.hosts.domain=127.0.0.1.nip.io \
     --set global.kas.enabled=true \
     -f examples/kind/values-base.yaml \
     -f examples/kind/values-no-ssl.yaml

3. ran bin/rails c in a webservice container, and checked the contents of Gitlab.config.gitlab_kas

Expected result: It should

1. be enabled
2. have external URL ws://kas.127.0.0.1.nip.io
3. have internal URL grpc://gitlab-kas.svc:8153

Result: ✅ with full content:

{"enabled"=>true,
 "secret_file"=>"/etc/gitlab/kas/.gitlab_kas_secret",
 "external_url"=>"ws://kas.127.0.0.1.nip.io",
 "internal_url"=>"grpc://gitlab-kas.svc:8153"}