Adds a new `main_clusterwide` connection option that can be configured
in `config/database.yml`. This connection is part of the
[Pods](https://gitlab.com/groups/gitlab-org/-/epics/7582) initiative
(soon to be renamed to "Silos").

This connection won't be configured in production or even GDK any time
soon and as such we've opted to have these code paths untested in CI.
Trying to test this new connection in CI would mean having a separate
`config/database.yml` that ran all RSpec tests again which is just too
expensive for this early stage of the project.

The `main_clusterwide` connection will eventually be used to have a
shared Postgres database that is used by many separate GitLab pods
(silos/instances). We need to share some data like settings, users,
licenses across all these pods while separating pod-specific data like
projects, notes, merge_requests . This allows us to horizontally scale
most of the data but still users can login and access da...