Design database infrastructure for Praefect

In order for Praefect to support replication coordination and failover it will rely on a PostgreSQL database. We must define a cost-effective and performant infrastructure to support this, with replication in mind.

More info on Praefect's data model: gitlab-org/gitaly#1495 (closed)

• https://ops.gitlab.net/gitlab-com/gitlab-com-infrastructure/-/merge_requests/1468
• https://ops.gitlab.net/gitlab-cookbooks/chef-repo/-/merge_requests/2816
• Opt-in to CloudSQL maintenance notifications (to be addressed on https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9462)
• Configure preferred maintenance window day of the week and time https://ops.gitlab.net/gitlab-com/gitlab-com-infrastructure/-/merge_requests/1480

/cc @gitlab-com/gl-infra for thoughts