Add pages project settings API

Add `/api/v4/projects/root%2Fhtml/pages` to return some basic pages
settings information for the given project:

```json
{
  "url": "http://html-root-4160ce5f0e9a6c90ccb02755b7fc80f5a2a09ffbb1976cf80b653.pages.gdk.test:3010",
  "is_unique_domain_enabled": true,
  "force_https": false,
  "is_deployed": true
}
```

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/436932

Changelog: added