Draft: Add domain verification in group-level

Splited this MR into smaller MRs:

• update-domain-verification-route !116621 (merged)
• update-project-select-component !116626 (merged)
• domain-verification-operation !116641 (merged)
• domain-verification-operation-views !116647 (merged)

What does this MR do and why?

Decouple the Pages wizard from domain verification by adding a new option at the group level: "Add Domain" on the Domain Verification screen.

The main controller and view files refer to Projects::PagesDomainsController. The main difference is that the group-level add domain form includes a project select component. The group domain verification feature still relies on the pages settings and the PagesDomain model/service.

Related to issue #375492 (closed)

design link Figma

Screenshots or screen recordings

Empty index page	Index page with domains

New domain page

Edit domain page

How to set up and validate locally

1. Upload Premium license to enable domain_verification feature
2. Enable pages in gitlab.yml:

  pages:
    enabled: true
    access_control: false
    host: example.com
    port: 80
    external_http: true
    external_https: true

3. Change application settings in rails console:

ApplicationSetting.current.update(pages_domain_verification_enabled: true, lets_encrypt_terms_of_service_accepted: true, lets_encrypt_notification_email: "test@example.com")

4. Visit domain verification page http://localhost:3000/groups/flightjs/-/settings/domain_verification
5. Click the Add Domain button.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @daveliu