Install pageinspect extension on the Container Registry PostgreSQL cluster

Context

We're performing an upgrade/migration of the GitLab.com container registry to a new platform backed by a metadata database (gitlab-org&5523 (closed)). During the percentage-based rollout (gitlab-org/container-registry#468 (closed)) we realized that the index bloat on the database has been steadily growing and approaching a threshold violation (gitlab-org/container-registry#497 (closed)).

As a followup from gitlab-org/container-registry#497 (comment 751787584), we should consider installing the pageinspect extension to enable inspecting the contents of database pages at a low level.