Migrate pages metadata when asked to serve virtual domain

What does this MR do?

This is another extraction from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30787 in order to break the later in smaller chunks.

This MR adds the ability to migrate pages metadata on-demand when asked to serve virtual domain. The background migration introduced will be used in the next (last) MR that will take care for the bulk migration.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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 to #28781 (closed).