Add gitlab-pages unique url lookup

What does this MR do and why?

⚠ depends on !109011 (merged)

Related to: #388179 (closed)

Enable gitlab pages to find sites by the unique domain.

Screenshots or screen recordings

Screen_Recording_2023-03-03_at_14.44.51

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Pre-conditions:

• Have gitlab-pages setup
• Have gitlab-runner setup

1. Enable the pages unique domain feature flag

   Feature.enable(:pages_unique_domain)

2. Create a new project using the Pages/Plain HTML template

3. Run the project pipeline to create a gitlab-pages deploy

4. Access the pages settings in Settings > Pages, like: http://gdk.test:3000/<project namespace>/pages

5. Enable Use unique domain and click in Save changes

6. Copy the URL in the Access pages > Your pages are served under: panel

7. Remove the protocol and port from the URL and add it in you /etc/hosts pointing to localhost, something like:

   # Unique Domain
   127.0.0.1  html-root-265cd6a7f146c5ae423a42292d2f66742749a886447617895e942.pages.gdk.test

8. You must be able to access the URL (with the protocol and port) in your browser, which mean that the lookup by the unique domain worked! 🎉

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.