Add `pg` (Postgres) to server components external packages. (#48402)
While exploring using
[Drizzle](https://github.com/drizzle-team/drizzle-orm/blob/main/drizzle-orm/src/pg-core/README.md#using-drizzle-orm-in-nextjs-app-router)
with the App Router, I noticed they have a callout for needing to
explicitly add `pg` to `serverComponentsExternalPackages`.
Adding `pg` here prevents manually needing to modify your configuration.