[KEYS-1166] pg create/update accept --connection-pool=none/pgbouncer

[breel-render]

Summary

Extend the pg get, update commands with --connection-pool

Motivation

https://linear.app/render-com/issue/KEYS-1166/opt-in-field-via-cli

How to verify

$ go run ./ pg get dpg-xyz-a
...
Connection pool: none
...

$ go run ./ pg update dpg-xyz-a --connection-pool=pgbouncer
Updated Postgres database

Changes:
  Connection pool:    none → pgbouncer
...

 $ go run ./ pg update dpg-xyz-a --connection-pool=none
Updated Postgres database

Changes:
  Connection pool:    pgbouncer → none
...

$ go run ./ pg get dpg-xyz-a
...
Connection pool: pgbouncer
...

Release notes

Manage the Render Postgres Connection Pool state