Certain users experienced Error 500s when loading projects from the
dashboard because the PostgreSQL query planner attempted to scan all
projects rather than just the user's authorized projects. This would
cause the query to hit a statement timeout.

To fix this, we add support for a CTE optimization fence to load
authorized projects first, which can be optionally used by
`ProjectsFinder` via the `use_cte` parameter. To be safe, we only enable
it for the finder call that loads the list of projects behind the
`use_cte_for_projects_finder` feature flag.

Closes https://gitlab.com/gitlab-org/gitlab/issues/198440