Lookup pages sites by the unique domain

Problem

Currently, cookies of different gitlab-pages sites under the same namespace are leaked due to the gitlab-pages default URL format: namespace.gitlab.io/project

Solution

based on the PoC !106961 (closed)

https://gitlab.com/gitlab-org/gitlab-pages/-/issues/820 added an option to use a unique domain for a gitlab-pages project. Now, we need to lookup gitlab-pages proejcts by this unique domain in the api/internal/pages.

How

The lookup in api/internal/pages is getting complex, so I propose to add a new finder, Pages::HostFinder to lookup the pages hosts.