Add support for custom domains to the Pages internal API

What does this MR do?

This is another extraction from https://gitlab.com/gitlab-org/gitlab-ce/issues/61927 in order break the later in smaller chunks.

This MR add support for custom pages domains to the endpoint introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32732. Namespaces domains are intentionally left out cause this is where the data migration and tricky queries will happen, this way we can introduce most of the code we'll eventually need before we add the last piece.

Keep in mind this branch is based on https://gitlab.com/gitlab-org/gitlab-ce/tree/61927-skeleton-pages-internal-api and can be merged only once https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32732 is merged.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related #61927 (moved)