Execute checks on external PostgreSQL instances

In the former GitLab Detective, we could connect to external PostgreSQL instances by constructing the connecting string manually. See https://gitlab.com/gitlab-com/support/toolbox/gitlab-detective-former/-/blob/main/roles/database_connection_check/tasks/main.yml#L70

GitLab Detective should be able to connect to external PostgreSQL instances and run the checks under https://gitlab.com/gitlab-com/support/toolbox/experimental/gitlab-detective-spot/-/tree/main/checks/database_primary_node for example. Because those checks are using gitlab-psql, maybe we need to transform this to /opt/gitlab/embedded/bin/psql along with a connection string.