Preload associations needed by Namespace#pages_virtual_domain

What does this MR do?

While looking at the performance if the /internal/pages API endpoint we noticed that there are couple N+1 queries problems:

• project_feature when eventually delegating private_pages? - https://gitlab.com/gitlab-org/gitlab/-/blob/335793d88dca35984d06fa2f62098e2bc3d09000/app/models/pages/lookup_path.rb#L16
• route when we call project.full_path - https://gitlab.com/gitlab-org/gitlab/-/blob/335793d88dca35984d06fa2f62098e2bc3d09000/app/models/pages/lookup_path.rb#L27

This MR updates Namespace#pages_virtual_domain to preload project_feature and route in order to avoid this.

Related to #207096 (closed).

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Does this MR meet the acceptance criteria?

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