Add option in GitLab Helm Chart to configure domain where users will access workspaces

MR:

• Add workspaces external URL config (!4473 - merged)
• Fix workspaces ingress hostname (!4525 - merged)

Description

Follow-up from Add workspaces related KAS options in GitLab He... (#6076 - closed), we need to expose a new domain to allow workspace access.

This should be done for all modes of deployment - GitLab Helm Chart, Omnibus, etc.

Acceptance criteria

• workspace's domain name should be accessible via Rails in CNG
• workspace's domain name should be accessible via Rails in Omnibus - It will be tracked in Add option in Omnibus to configure domain where... (gitlab-org/omnibus-gitlab#9335)