Enable possibility to use non-Pooled connection for Emmett Postgres projections

[oskardudycz]

It's a similar case to event-driven-io/Pongo#35. To support cases like Cloudflare Hyperdrive or Supabase that has their own pooling we need to allow injecting regular connection.

Part of that is already available in Pongo from event-driven-io/Pongo#38. I'd like to finish first abstractions in event-driven-io/Pongo#40, and then build handling based on that.